Help with illusion rules

[Bobthewizard]

On forums, it seems that sometimes players try to do things with illusions that don’t seem to fall in the spell description, and a lot of DMs seem to arbitrarily nerf illusions by ignoring them or placing more restrictions on them. There isn’t much sage advice governing illusions. I think they purposefully keep it open to DM interpretation, so I tried to come up with guidelines for my players so they have a better idea of what to expect from their illusion spells. Here they are. Let me know what you would change.

Illusion Rules for minor illusion, silent image and major image (edited to specify spells)

1. Only objects, images or visible phenomena – cannot use illusions to make something invisible or create a window or hole. You make a painting of a tunnel, but not a tunnel. Cannot make the room seem empty. (Edit to add): Images will have basic shadows and reflections but cannot darken a room or create a functioning mirror.

2. Entire illusion must fit in the area. Cannot use optical illusions to make illusions seem bigger.

3. Physical interaction does not reveal an illusion if it is something that interaction normally wouldn’t affect. According to the spell description, it’s the unexpected passing through that reveals the illusion. Darkness, ghosts and fog are not revealed with interaction and still require an INVESTIGATION check. (edited to remove darkness due to conversation below)

4. Allies told about the illusion can choose to ignore it but can’t see through it until they interact with it or use their action to pass an INVESTIGATION check.

5. For NPCs and monsters, investigation will fall into one of 3 categories.

a. If the illusion appears reasonable and target has more pressing concerns, they believe it and do not try to save.

b. If the illusion is out of place or their most pressing concern, target will use their action to make an INVESTIGATION check or interact with the illusion

c. If unsure, DM will use passive PERCEPTION to decide if the target uses action to save. If they pass PERCEPTION check, they think something might not be right and want to investigate. DM can role this for players, telling them “you think it might be an illusion.” describing something that might be wrong with the illusion. (edited to make more immersive)

[Chronos]

All of this seems fine to me, but I should warn you:


2. Entire illusion must fit in the area. Cannot use optical illusions to make illusions seem bigger.
If you have a Certain Type of player, they'll try to figure out exactly how this works. For instance, if you put an illusion of a mirror in a small room, does it work as a mirror? Is what it reflects fixed (i.e., it'll show a reflection of an empty room), or will it accurately show someone looking in it that they have a smudge of dirt on their left cheek? What happens if someone opens up a door in that small room, and the small room is now a big one? Does the image of the reflection just stop abruptly at the size limit of the illusion?

I refrain from asking such questions around the table, because out of my group of seven, probably only two of us would enjoy the ensuing debate. But I would enjoy that debate, and some other players might not have my level of self-restraint. My personal ruling would be that low-level illusion spells are incapable of making any reflective surface of sufficient fidelity to act as a mirror (and the lack of such faithful reflections, on something other than a mirror, might be one of the details that can be picked up with an Investigation check). You might still get things like sunlight glinting off of the edge of a sword or shield, but it won't behave in quite the same way that real glints do.

[Bobthewizard]

My personal ruling would be that low-level illusion spells are incapable of making any reflective surface of sufficient fidelity to act as a mirror (and the lack of such faithful reflections, on something other than a mirror, might be one of the details that can be picked up with an Investigation check). You might still get things like sunlight glinting off of the edge of a sword or shield, but it won't behave in quite the same way that real glints do.

I agree with you and this is what I was trying to say. The other question we came up with is if I am in a 100' room with a single fire for a light source, does enclosing the fire in a brick wall block light outside the effect of the spell? I would rule no. You can't see the fire but outside the illusion parameters the room is still lit. Which seems weird, but otherwise the illusion is getting much greater area.

[HappyDaze]

Illusion Rules

1. Only objects, images or visible phenomena – cannot use illusions to make something invisible or create a window or hole. You make a painting of a tunnel, but not a tunnel. Cannot make the room seem empty.

3. Physical interaction does not reveal an illusion if it is something that interaction normally wouldn’t affect. According to the spell description, it’s the unexpected passing through that reveals the illusion. Darkness, ghosts and fog are not revealed with interaction and still require an INVESTIGATION check.



If you cannot make a room seem empty, then you cannot make it seem absent of light. Darkness is not a visible phenomena, it s the absence of visible phenomena. However, your other alternatives like fog (or smoke or mist) would work fine.

[Bobthewizard]

If you cannot make a room seem empty, then you cannot make it seem absent of light. Darkness is not a visible phenomena, it s the absence of visible phenomena. However, your other alternatives like fog (or smoke or mist) would work fine.

Good point. I like your ruling too. It's more consistent. I also wasn't comfortable with a 1st level Silent Image being able to replicate a 2nd level Darkness spell, but when I read the rule I thought it would be able to. Thanks.

Oh, so does the image even make a shadow? Maybe my brick wall around the fire example above wouldn't even make it dark in the area of the spell.

[Segev]

On forums, it seems that sometimes players try to do things with illusions that don’t seem to fall in the spell description, and a lot of DMs seem to arbitrarily nerf illusions by ignoring them or placing more restrictions on them. There isn’t much sage advice governing illusions. I think they purposefully keep it open to DM interpretation, so I tried to come up with guidelines for my players so they have a better idea of what to expect from their illusion spells. Here they are. Let me know what you would change.

Illusion Rules

1. Only objects, images or visible phenomena – cannot use illusions to make something invisible or create a window or hole. You make a painting of a tunnel, but not a tunnel. Cannot make the room seem empty.Reasonable.


2. Entire illusion must fit in the area. Cannot use optical illusions to make illusions seem bigger. Two things: do you control orientation of the area? i.e., if I want a longer-than-ten-foot illusory pole, can I orient my "10 ft. cube" to allow me to extend the pole from corner to far corner in an arbitrary direction? If so, this leads to some interesting things that can be done without, in my opinion, breaking anything.

The other question is...what do you mean by "optical illusions?" I ask because your example in (1) about making a painting of a tunnel is essentially an optical illusion of something that extends outside the area.


3. Physical interaction does not reveal an illusion if it is something that interaction normally wouldn’t affect. According to the spell description, it’s the unexpected passing through that reveals the illusion. Darkness, ghosts and fog are not revealed with interaction and still require an INVESTIGATION check.Reasonable.


4. Allies told about the illusion can choose to ignore it but can’t see through it until they interact with it or use their action to pass an INVESTIGATION check. Also reasonable.


5. For NPCs and monsters, investigation will fall into one of 3 categories.

a. If the illusion appears reasonable and target has more pressing concerns, they believe it and do not try to save.

b. If the illusion is out of place or their most pressing concern, target will use their action to make an INVESTIGATION check or interact with the illusionDoes this mean that, if the illusion is "their most pressing concern," they'll stop to investigate it rather than reacting to it as if it were real in the first place? e.g., a major image of a wolf is menacing them; do they actually take a turn to make an investigation check, or do they try to attack it? How do you judge whether they hit the wolf illusion and thus interacted, or missed it and thus didn't?


c. If unsure, DM will use passive PERCEPTION to decide if the target uses action to save. If they pass PERCEPTION check, they think something might not be right and want to investigate. DM can role this for players, telling them “you think it might be an illusion.”I suppose this is a reasonable way for a DM to make a decision whether his NPC investigates or not if the DM is otherwise ambivalent about it.


If you cannot make a room seem empty, then you cannot make it seem absent of light. Darkness is not a visible phenomena, it s the absence of visible phenomena. However, your other alternatives like fog (or smoke or mist) would work fine.
Magical darkness, then.

[Bobthewizard]

Two things: do you control orientation of the area? i.e., if I want a longer-than-ten-foot illusory pole, can I orient my "10 ft. cube" to allow me to extend the pole from corner to far corner in an arbitrary direction? If so, this leads to some interesting things that can be done without, in my opinion, breaking anything.

Absolutely, a 15' silent image can make a 21' wall or a 25' pole


The other question is...what do you mean by "optical illusions?" I ask because your example in (1) about making a painting of a tunnel is essentially an optical illusion of something that extends outside the area.

You would never run into a painting of a tunnel. As you approach it, it wouldn't act the same as an actual tunnel.


Does this mean that, if the illusion is "their most pressing concern," they'll stop to investigate it rather than reacting to it as if it were real in the first place? e.g., a major image of a wolf is menacing them; do they actually take a turn to make an investigation check, or do they try to attack it? How do you judge whether they hit the wolf illusion and thus interacted, or missed it and thus didn't?

I suppose this is a reasonable way for a DM to make a decision whether his NPC investigates or not if the DM is otherwise ambivalent about it.

This is the farthest from the actual rules but here's what I do. I say that attacking the illusion is using your action to investigate it. The character doesn't have to say "I'm going to investigate to see if this wolf is an illusion". They just attack it. If the character attacks the illusion, I treat the attack role as their investigation check, telling them they missed if they fail. If they hit (pass the investigation check), as long as it isn't a ghost, they'd see their weapon go through the illusion and know it is not real. For NPCs, as long as they aren't using their action to do something else, I would count it as using their action to investigate. Edit: If it's a stationary illusion, I'd just have the attack be their interact with object action and let them know it's an illusion without rolling.

[Segev]

Absolutely, a 15' silent image can make a 21' wall or a 25' poleCool, works for me.


You would never run into a painting of a tunnel. As you approach it, it wouldn't act the same as an actual tunnel. Forced perspective might make a 15' cube look like a 30' long hall, though. I guess what I'm asking is, what is it you're disallowing, and why? I'm not sure what it is about "optical illusions" you're saying isn't okay.


This is the farthest from the actual rules but here's what I do. I say that attacking the illusion is using your action to investigate it. The character doesn't have to say "I'm going to investigate to see if this wolf is an illusion". They just attack it. If the character attacks the illusion, I treat the attack role as their investigation check, telling them they missed if they fail. If they hit (pass the investigation check), as long as it isn't a ghost, they'd see their weapon go through the illusion and know it is not real. For NPCs, as long as they aren't using their action to do something else, I would count it as using their action to investigate.
That's fair, and seems a decent house rule to work it out. So if they fail the Investigation check against the illusory wolf, they assume they "missed" when they attacked it, right?

How do you plan to handle illusions that lack particular sensory inputs? A silent image of a wolf, for instance, has the same Investigation DC as a major image cast by the same caster.

[NNescio]

1. Only objects, images or visible phenomena – cannot use illusions to make something invisible or create a window or hole. You make a painting of a tunnel, but not a tunnel. Cannot make the room seem empty.

I agree. That said there's something odd about how Minor Illusion explicitly calls out "footprints" as an example of an object, despite them being, well, empty space (indentations). I sort of rationalize it away by saying maybe minor indentations on a surface count but not overt empty space.


2. Entire illusion must fit in the area. Cannot use optical illusions to make illusions seem bigger.

I'd allow optical illusions, but the player has to specify what they're trying to achieve (so no arbitrary I-try-to-fit-a-40'-cube-in-a-20'-cube-space), and it would only work from certain angles, due to perspective. So, a painting of a window would work when seen from the front, at a distance, but would fail when somebody sees it from the side.



3. Physical interaction does not reveal an illusion if it is something that interaction normally wouldn’t affect. According to the spell description, it’s the unexpected passing through that reveals the illusion. Darkness, ghosts and fog are not revealed with interaction and still require an INVESTIGATION check. (edited to remove darkness due to conversation below)

Sounds fair, and is probably RAI. Though by a strict reading of RAW, I'd say the interaction would still reveal the illusion even if you normally expect things to pass through whatever it was faking.



4. Allies told about the illusion can choose to ignore it but can’t see through it until they interact with it or use their action to pass an INVESTIGATION check.
I would apply it to the caster too, for most illusions (the Image line, that is).



5. For NPCs and monsters, investigation will fall into one of 3 categories.

a. If the illusion appears reasonable and target has more pressing concerns, they believe it and do not try to save.

b. If the illusion is out of place or their most pressing concern, target will use their action to make an INVESTIGATION check or interact with the illusion

c. If unsure, DM will use passive PERCEPTION to decide if the target uses action to save. If they pass PERCEPTION check, they think something might not be right and want to investigate. DM can role this for players, telling them “you think it might be an illusion.”

Agreed, but I wouldn't be that explicit. I would probably tell them something along the lines of "you feel like there's something off about the tiles on this wall", "you feel a breeze where there shouldn't be any", or "there's something odd about the way that armor stand reflects the light".



If you have a Certain Type of player, they'll try to figure out exactly how this works. For instance, if you put an illusion of a mirror in a small room, does it work as a mirror? Is what it reflects fixed (i.e., it'll show a reflection of an empty room), or will it accurately show someone looking in it that they have a smudge of dirt on their left cheek? What happens if someone opens up a door in that small room, and the small room is now a big one? Does the image of the reflection just stop abruptly at the size limit of the illusion?

I would say it would like it's reflecting light, and most casual observers will see what they expect to see, but more perceptive ones might notice something off about it, warranting them to make an active Investigation check.


I agree with you and this is what I was trying to say. The other question we came up with is if I am in a 100' room with a single fire for a light source, does enclosing the fire in a brick wall block light outside the effect of the spell? I would rule no. You can't see the fire but outside the illusion parameters the room is still lit. Which seems weird, but otherwise the illusion is getting much greater area.

I'll rule it looks like it blocks the light, but the room is still lit, oddly.


If you cannot make a room seem empty, then you cannot make it seem absent of light. Darkness is not a visible phenomena, it s the absence of visible phenomena. However, your other alternatives like fog (or smoke or mist) would work fine.

You can make a black sphere, which will sort of look like the Darkness spell, or solid black paintings/curtains/props that can make areas appear dark from a distance. Both wouldn't actually block illumination though, so it would seem weird, especially when people turn to the side and are not looking at the illusion directly.

But yeah, no actual darkness.


Oh, so does the image even make a shadow? Maybe my brick wall around the fire example above wouldn't even make it dark in the area of the spell.

It appears to cast a shadow. Otherwise all illusions will be blatantly fake as they would look flat (no 'dynamic' shading). How real the shadow is, well, I'd say it's abstracted away by the Spell DC.

The shadow won't be visible if you're looking away from the illusion though, so again, this clues in a perspective observer that something is off.

[Bobthewizard]

Cool, works for me.

Forced perspective might make a 15' cube look like a 30' long hall, though. I guess what I'm asking is, what is it you're disallowing, and why? I'm not sure what it is about "optical illusions" you're saying isn't okay.

What I'm trying to disallow is the "I make an illusion of a tunnel or pit so the enemy runs into it." If you can describe an optical illusion I might allow it, but just know it would only be from one perspective. Getting close enough or moving across in front of it would reveal the illusion without a check.


I agree. That said there's something odd about how Minor Illusion explicitly calls out "footprints" as an example of an object, despite them being, well, empty space (indentations). I sort of rationalize it away by saying maybe minor indentations on a surface count but not overt empty space.

As for footprints, I'd allow muddy footprints in a hallway but not footprints in mud.


I'd allow optical illusions, but the player has to specify what they're trying to achieve (so no arbitrary I-try-to-fit-a-40'-cube-in-a-20'-cube-space), and it would only work from certain angles, due to perspective. So, a painting of a window would work when seen from the front, at a distance, but would fail when somebody sees it from the side.

Agree. See above


Sounds fair, and is probably RAI. Though by a strict reading of RAW, I'd say the interaction would still reveal the illusion even if you normally expect things to pass through whatever it was faking.

That's how I thought before too. But all of them qualify the interaction clause as follows, "Physical interaction with the image reveals it to be an illusion, because things can pass through it." So I'm not sure about RAW or RAI. The important thing is having the DM and players agree ahead of time.



I would apply it to the caster too, for most illusions (the Image line, that is).

Brutal. But acceptable to me. :smallsmile:


Agreed, but I wouldn't be that explicit. I would probably tell them something along the lines of "you feel like there's something off about the tiles on this wall", "you feel a breeze where there shouldn't be any", or "there's something odd about the way that armor stand reflects the light".

I love this just for the immersion. Players will get the hint and be able to see it from the characters point of view better.


I would say it would like it's reflecting light, and most casual observers will see what they expect to see, but more perceptive ones might notice something off about it, warranting them to make an active Investigation check.

I'll rule it looks like it blocks the light, but the room is still lit, oddly.

You can make a black sphere, which will sort of look like the Darkness spell, or solid black paintings/curtains/props that can make areas appear dark from a distance. Both wouldn't actually block illumination though, so it would seem weird, especially when people turn to the side and are not looking at the illusion directly.

But yeah, no actual darkness.

It appears to cast a shadow. Otherwise all illusions will be blatantly fake as they would look flat (no 'dynamic' shading). How real the shadow is, well, I'd say it's abstracted away by the Spell DC.

The shadow won't be visible if you're looking away from the illusion though, so again, this clues in a perspective observer that something is off.

This is all hard. I'd allow shadows and blocking the light source but not darkness. I like your takes on illusions. I'll probably incorporate a lot of this before I give it to my players.

[Segev]

All of that sounds reasonable; I could work with it if I were one of your players. As far as "footprints" from minor illusion, remember that you can add things. So having the "ground" be raised by the depth of the footprints and having footprints in that is viable. If they're close enough to touch the footprints, they are already making an Investigation check, anyway, and probably will touch them.



Personally, I treat illusions as projecting an intangible object/creature/phenomenon. Illusory walls still block light/cast shadows. Illusory mirrors reflect just fine. Light and shadows play off of them the way they would real materials. Investigation checks reveal flaws in its interaction with the environment, or subtle missing cues, or the like.

[HappyDaze]

I agree. That said there's something odd about how Minor Illusion explicitly calls out "footprints" as an example of an object, despite them being, well, empty space (indentations). I sort of rationalize it away by saying maybe minor indentations on a surface count but not overt empty space.


They could be bloody footprints, or mud tracked across the floor, but otherwise you don't create an illusion of footprints, you create an illusion of a think coating of dust/sand/frost/something over the entire floor except for a set of foot-shaped holes in the illusion. In the latter case, Investigation might reveal that characters following those footprints don't leave footprints of their own...

[Bobthewizard]

Personally, I treat illusions as projecting an intangible object/creature/phenomenon. Illusory walls still block light/cast shadows. Illusory mirrors reflect just fine. Light and shadows play off of them the way they would real materials. Investigation checks reveal flaws in its interaction with the environment, or subtle missing cues, or the like.

I like all of that. I'm not really trying to get into the weeds on reflections and shadows. I'm trying to dissuade illusions of windows, pits, and tunnels as well as trying to use it for invisibility and now darkness. It's not that hard to make an illusion of something you can hide behind.


They could be bloody footprints, or mud tracked across the floor, but otherwise you don't create an illusion of footprints, you create an illusion of a think coating of dust/sand/frost/something over the entire floor except for a set of foot-shaped holes in the illusion. In the latter case, Investigation might reveal that characters following those footprints don't leave footprints of their own...

For the thick coating over the floor, up to the area of the spell of course.

[Tanarii]

My ruling is that physical interaction lets people (who observe it) know it is an illusion, but doesn't make it go faint. Only the investigation check does that. This includes the caster.

This ruling means I can be fairly strict on things like no invisible holes in solid objects, what counts as an object, etc, while still making illusions fairly powerful.

Edit: it's based on a bit of parsing of the rules, treating the physical interaction clause as a seperate clause, and the investigation / go faint clauses as a single clause.

[Segev]

The rules are pretty clear that, unless the spell says it can make empty space (like hallucinatory terrain and mirage arcane do regarding rendering difficult terrain like a forest into a neat and empty field), it creates an image, not renders something invisible. Note that the spells that do some of both tend to talk about altering something's appearance, rather than creating an image or sound or the like. (Disguise self, for instance, can make parts of you invisible, and the example in the spell itself illustrates this when it talks about making yourself skinnier than you are.)

[Bobthewizard]

My ruling is that physical interaction lets people (who observe it) know it is an illusion, but doesn't make it go faint. Only the investigation check does that. This includes the caster.

This ruling means I can be fairly strict on things like no invisible holes in solid objects, what counts as an object, etc, while still making illusions fairly powerful.

Edit: it's based on a bit of parsing of the rules, treating the physical interaction clause as a seperate clause, and the investigation / go faint clauses as a single clause.

I don't think I agree with that parsing of the rules, but it's fine to use it if it works at your table. I think the most important part is to say, "here's what you can do with illusions, and if you follow this, I'll try to let them work on the monsters using these guidelines."


The rules are pretty clear that, unless the spell says it can make empty space (like hallucinatory terrain and mirage arcane do regarding rendering difficult terrain like a forest into a neat and empty field), it creates an image, not renders something invisible. Note that the spells that do some of both tend to talk about altering something's appearance, rather than creating an image or sound or the like. (Disguise self, for instance, can make parts of you invisible, and the example in the spell itself illustrates this when it talks about making yourself skinnier than you are.)

Good point. Thanks for pointing out the differences with other illusions. I was focusing on minor illusion, silent/major image and hadn't really thought about disguise self and mirage arcane.

[comk59]

As for footprints, I'd allow muddy footprints in a hallway but not footprints in mud.


Okay, but what if the illusion is a quarter inch of mud with footprints in it, layered directly above the regular mud?

[Eldariel]

For attacks, if the illusion is able to move, I just treat it as a touch attack of sorts with the illusion using the caster's spellcasting modifier as its "dex" bonus (so 10+Casting Stat as its AC).

[Keravath]

Unless an illusion calls out that it is malleable or adjustable over time then it is creating a static image. So you could create an illusion with a mirror using Silent Image but the image in the mirror would not change.

However, I tend to rule that if you could create a picture of something then you can make an illusion. Creating an illusion of muddy tracks on stone or tracks in mud are the same process. They create an image that overlays the reality of what is actually there. You could put up an illusion of a wall making a room smaller.

The main area of contention is creating an image that portrays something in the distance that covers something nearby effectively rendering it invisible. I tend to think that the lack of real perspective in such an illusion might make it obvious that it is an illusion thus making it impossible to hide something behind an illusion of empty space (or rather an illusion of objects at a distance). You could create an illusion of a crate and then hide behind it though.

[Tanarii]

I don't think I agree with that parsing of the rules, but it's fine to use it if it works at your table. I think the most important part is to say, "here's what you can do with illusions, and if you follow this, I'll try to let them work on the monsters using these guidelines."Right, not everyone does. That's why I explained the ruling first since it's the important part, and only then how I've justified it. :smallamused:

[JackPhoenix]

This is the farthest from the actual rules but here's what I do. I say that attacking the illusion is using your action to investigate it. The character doesn't have to say "I'm going to investigate to see if this wolf is an illusion". They just attack it. If the character attacks the illusion, I treat the attack role as their investigation check, telling them they missed if they fail. If they hit (pass the investigation check), as long as it isn't a ghost, they'd see their weapon go through the illusion and know it is not real. For NPCs, as long as they aren't using their action to do something else, I would count it as using their action to investigate.

It's not far from the rules, actually. The player says what he wants the character to do, the GM decided how to resolve it. Deciding that attacking an illusion is actually worthy of investigate check instead of an attack roll is entirely possible.

[tieren]

I like all of that. I'm not really trying to get into the weeds on reflections and shadows. I'm trying to dissuade illusions of windows, pits, and tunnels as well as trying to use it for invisibility and now darkness. It's not that hard to make an illusion of something you can hide behind.


I had an illusionist with a DM that felt similarly about the negative space illusions. [I personally have sen weird art in the real wold which creates the illusion of holes in the sidewalks or walls that I believe and have a different opinion of what could then be created with magic]

We worked out a compromise that worked devilishly well for my character's personality. I carried around a satchel of parchments prepainted with a particularly obvious symbol or the message "this is an illusion". Then everytime I created an illusion I would include the image of an identical parchment on the illusion itself (so a wall with an "this is an illusion" sign for example). After an encounter or two when the enemy figured out it wasn't a trick and they were an illusion I started tacking them up or dropping them n the floor as we moved about. So instead of the illusion of a pit trap I could leave an illusion of a sign saying "this is a pit trap" or for the illiterate the calling card symbol of my illusions. Then the enemy has to make the choice to investigate or just walk across and hope I didn't create the illusion of a floor over an actual pit trap, or maybe they run into a real wall I tacked an illusion parchment to, or I could just create the illusion of the parchment, etc.

It accomplished my goal of slowing down pursuers and forcing checks, and became a heck of a lot of fun with a castle full of enemies who couldn't figure out what was real and what wasn't.

[Segev]

That's a fun and clever way to abuse knowledge that an illusionist is about, yeah.

I had an illusionist with a DM that felt similarly about the negative space illusions. [I personally have sen weird art in the real wold which creates the illusion of holes in the sidewalks or walls that I believe and have a different opinion of what could then be created with magic]

Such art tends to only work from partiuclar perspectives. If you see it IRL, rather than from a carefully-selected camera angle for a still, it still looks good from the right perspective, but moving from that viewing angle quickly distorts the image. You can actually see still shots taken from other angles on some of those cool drawings for comparison, and they're often actually hard to even tell what they're supposed to be, due to distortion.

So, were I DMing, I'd let you create your "illusion of empty space" using these tricks, but you're essentially painting a perspective-drawing, and that's going to only work from a particular viewing angle and distance. It will be painfully obvious that it's "painted on" the surface you're trying to make an illusory hole into from any other angle, even if they don't Investigate to tell the "painting" is an illusion.

[NaughtyTiger]

My ruling is that physical interaction lets people (who observe it) know it is an illusion, but doesn't make it go faint. Only the investigation check does that. This includes the caster.

I like this.

I get annoyed when i spend a whole action to investigate the illusion (and risk not meeting the DC), but the fighter can burn 1 of 3 attacks to automatically see through it.

[tieren]

That's a fun and clever way to abuse knowledge that an illusionist is about, yeah.


Such art tends to only work from partiuclar perspectives. If you see it IRL, rather than from a carefully-selected camera angle for a still, it still looks good from the right perspective, but moving from that viewing angle quickly distorts the image. You can actually see still shots taken from other angles on some of those cool drawings for comparison, and they're often actually hard to even tell what they're supposed to be, due to distortion.

So, were I DMing, I'd let you create your "illusion of empty space" using these tricks, but you're essentially painting a perspective-drawing, and that's going to only work from a particular viewing angle and distance. It will be painfully obvious that it's "painted on" the surface you're trying to make an illusory hole into from any other angle, even if they don't Investigate to tell the "painting" is an illusion.

I agree they wouldn't hold up long or from other perspectives, but in most situations where I would want to use this, the perspective IS controlled, for example, to slow pursuers I might put an illusion of a hole in the floor which they will see straight on from the perspective of someone coming down that corridor. My goal isn't to stop them so they find some other way, but to cause them to stop for a moment to investigate it before continuing pursuit. I don't expect them to stand up to scrutiny.

[HappyDaze]

Unless an illusion calls out that it is malleable or adjustable over time then it is creating a static image. So you could create an illusion with a mirror using Silent Image but the image in the mirror would not change.

So a reflective surface would not change what is being reflected? The reflection is how light plays against it, not the illusion itself. If you do this, then an illusion in a lit room is going to be just as bright and vivid when the room goes dark and that's just silly.

[JackPhoenix]

I agree they wouldn't hold up long or from other perspectives, but in most situations where I would want to use this, the perspective IS controlled, for example, to slow pursuers I might put an illusion of a hole in the floor which they will see straight on from the perspective of someone coming down that corridor. My goal isn't to stop them so they find some other way, but to cause them to stop for a moment to investigate it before continuing pursuit. I don't expect them to stand up to scrutiny.

If you're pursuing someone, turn behind corner (I'll assume the caster isn't trying to use the illusion while the pursuer can see him) and see a pit in the distance, you won't stop to gawk at the pit from 20' away. You'll move closer to the "pit", the way you would move anyway, and notice it's not actually a hole as your perspective change. No need to stop or investigate, it would be obvious. Those forced perspective tricks don't work from a short distance.

[E’Tallitnics]

On forums, it seems that sometimes players try to do things with illusions that don’t seem to fall in the spell description, and a lot of DMs seem to arbitrarily nerf illusions by ignoring them or placing more restrictions on them. There isn’t much sage advice governing illusions. I think they purposefully keep it open to DM interpretation, so I tried to come up with guidelines for my players so they have a better idea of what to expect from their illusion spells. Here they are. Let me know what you would change.

Illusion Rules

1. Only objects, images or visible phenomena – cannot use illusions to make something invisible or create a window or hole. You make a painting of a tunnel, but not a tunnel. Cannot make the room seem empty.

2. Entire illusion must fit in the area. Cannot use optical illusions to make illusions seem bigger.

3. Physical interaction does not reveal an illusion if it is something that interaction normally wouldn’t affect. According to the spell description, it’s the unexpected passing through that reveals the illusion. Darkness, ghosts and fog are not revealed with interaction and still require an INVESTIGATION check. (edited to remove darkness due to conversation below)

4. Allies told about the illusion can choose to ignore it but can’t see through it until they interact with it or use their action to pass an INVESTIGATION check.

5. For NPCs and monsters, investigation will fall into one of 3 categories.

a. If the illusion appears reasonable and target has more pressing concerns, they believe it and do not try to save.

b. If the illusion is out of place or their most pressing concern, target will use their action to make an INVESTIGATION check or interact with the illusion

c. If unsure, DM will use passive PERCEPTION to decide if the target uses action to save. If they pass PERCEPTION check, they think something might not be right and want to investigate. DM can role this for players, telling them “you think it might be an illusion.”

Here's a 1 hour discussion on how a DM can adjudicate illusions in 5e: https://youtu.be/6l51s0GFflY

[Segev]

I agree they wouldn't hold up long or from other perspectives, but in most situations where I would want to use this, the perspective IS controlled, for example, to slow pursuers I might put an illusion of a hole in the floor which they will see straight on from the perspective of someone coming down that corridor. My goal isn't to stop them so they find some other way, but to cause them to stop for a moment to investigate it before continuing pursuit. I don't expect them to stand up to scrutiny.


If you're pursuing someone, turn behind corner (I'll assume the caster isn't trying to use the illusion while the pursuer can see him) and see a pit in the distance, you won't stop to gawk at the pit from 20' away. You'll move closer to the "pit", the way you would move anyway, and notice it's not actually a hole as your perspective change. No need to stop or investigate, it would be obvious. Those forced perspective tricks don't work from a short distance.

Yeah, unfortunately for the perspective tricks, it stops working long before you ahve to slow up to investigate. It won't look convincing enough to be problematic from five feet away, which is where you'd have to stop to investigate a hole. They can pretty much run up to it, slow for so little time that it won't be reflected in the mechanics of movement as their brains adjust from "I thought that was a hole" to "obviously something painted on the ground" and then continue on at full clip. You'd be served better by a false wall or rockfall or the like.

[Nagog]

So I'm going to play Devil's Advocate here, see if I can stir up some good discussion: If I were to make an illusion of a small, pitch-black sheet around somebody's eyes, would they be blinded? Could I make a similar illusion around a window or doorway to make it seem dark, without actual darkness?
The physics behind it is more of, if light can pass through the illusion (as per the campfire and the wall example), and all sight is is light interacting with our eyes, does the illusion produce/dim the light around it to produce color? Or in another way entirely, is the illusion psychically projected into the mind of the observer, making it similar to (but not the same as) Enchantment spells?

[HappyDaze]

So I'm going to play Devil's Advocate here, see if I can stir up some good discussion: If I were to make an illusion of a small, pitch-black sheet around somebody's eyes, would they be blinded? Could I make a similar illusion around a window or doorway to make it seem dark, without actual darkness?
The physics behind it is more of, if light can pass through the illusion (as per the campfire and the wall example), and all sight is is light interacting with our eyes, does the illusion produce/dim the light around it to produce color? Or in another way entirely, is the illusion psychically projected into the mind of the observer, making it similar to (but not the same as) Enchantment spells?

Is your illusion able to move with their head? This doesn't negate the questions you asked, but it might make them irrelevant.

[Nagog]

Is your illusion able to move with their head? This doesn't negate the questions you asked, but it might make them irrelevant.

True, which is also why I added in the Window. I thought placing it on somebody's eyes would be cool, but unless they're completely stationary it wouldn't last past the recoil of suddenly being blinded.

[Chronos]

One situation where you could make a false-perspective illusion and keep it effective would be if you expect someone to be looking through a keyhole. Though that's obviously a very limited example.


Quoth HappyDaze:

So a reflective surface would not change what is being reflected? The reflection is how light plays against it, not the illusion itself. If you do this, then an illusion in a lit room is going to be just as bright and vivid when the room goes dark and that's just silly.
The problem with that is that as soon as you can create mirrors or lenses that behave like the real thing, you can completely bypass all of the size limits on illusions. You could arrange mirrors in a 5' cube in such a way that you produce an illusion of arbitrary size, and so make a castle from a cantrip.

[Xetheral]

The physics behind it is more of, if light can pass through the illusion (as per the campfire and the wall example), and all sight is is light interacting with our eyes, does the illusion produce/dim the light around it to produce color? Or in another way entirely, is the illusion psychically projected into the mind of the observer, making it similar to (but not the same as) Enchantment spells?

The physics interpretation that I prefer is that illusions don't reflect or absorb photons at all. Instead, any photon passing through the illusion is "painted" by the illusion. When a "painted" photon hits a photosensitive cell of a creature who hasn't disbelieved the illusion, that creature sees the "painted" frequency rather than the frequency the cell is sensitive to. This makes the illusion visible even though photons aren't bouncing off it.

The "paint" can survive reflection, but not absorption/re-emission, so illusions are visible in mirrors. But since illusions can't reflect light, they can't themselves show reflections (unless the illusion can be changed on the fly by the illusionist). Additionally, this interpretation makes illusions appear exactly as bright as whatever is behind them (which fits the prohibition in most illusions against creating light). In some extreme cases (e.g. highly directional or patterned light sources) this may make illusions immediately obvious. But in most cases the discrepancies in lighting will be subtle, and noticing those discrepancies is exactly what the investigation check represents.

I find this interpretation neatly answers most of the questions relating to illusions, reflections, and light sources, at the downside of making adjudicating illusions require the DM to have some knowledge of optics. It also produces some oddities of its own when viewing illusions through colored, translucent materials, but that rarely comes up (and is a fun thought exercise when it does).

Example: A mage on a castle wall casts an illusion of a dragon between the wall and some marauding bandits. It is dusk, and the castle wall is lit with torches. In the dim, pervasive light of dusk the dragon appears uniformly lit exactly as if it were real, because the background is uniformly lit. Those who take the time to study the dragon, however, and succeed on the investigation check, notice the subtle brighter spots on the dragon that appear whenever the dragon occludes one of the torches on the wall. After a successful check, they can see both the original photon and the '"paint", permitting then to see through the illusion.

[HappyDaze]

As a simplicity hand wave, how much does it break things if visual illusions are just assumed to create "seemingly real but totally immaterial" objects/creatures (in that they visually act just like the real things until discovered to be false through Investigation)?

[Segev]

So I'm going to play Devil's Advocate here, see if I can stir up some good discussion: If I were to make an illusion of a small, pitch-black sheet around somebody's eyes, would they be blinded? Could I make a similar illusion around a window or doorway to make it seem dark, without actual darkness?
The physics behind it is more of, if light can pass through the illusion (as per the campfire and the wall example), and all sight is is light interacting with our eyes, does the illusion produce/dim the light around it to produce color? Or in another way entirely, is the illusion psychically projected into the mind of the observer, making it similar to (but not the same as) Enchantment spells?
I allow it to block light. This rarely causes any problems, given the proliferation of Darkvision. And a torch or the like is easy enough to light. If the image is translucent due to making the investigation check, the. The viewer can see through it.

If you don’t let it block light, you wind up with really weird questions about light sources behind them.


One situation where you could make a false-perspective illusion and keep it effective would be if you expect someone to be looking through a keyhole. Though that's obviously a very limited example.


The problem with that is that as soon as you can create mirrors or lenses that behave like the real thing, you can completely bypass all of the size limits on illusions. You could arrange mirrors in a 5' cube in such a way that you produce an illusion of arbitrary size, and so make a castle from a cantrip.
Mirror tricks like this are like any forced perspective illusion. You’re still getting only a five foot cube that looks like a window into a larger space than it is from a particular angle. There’s a reason fun houses and the like are very controlled environments.

[Chronos]

Ooh, Xetheral, I'm totally stealing that. I'll have to think about edge cases for a while, but I think that manages to completely account for all of the behaviors of illusions, without breaking anything.

HappyDaze, that's what I just answered four posts ago.

EDIT: Yes, Segev, it's still only for a limited range of viewing positions, but much less limited than forced-perspective tricks. Make your arrangement of mirrors and lenses elaborate enough, and a 5' cube full of optics could look like a window opening on a vast landscape, from any angle through which a viewer could look through the window. To look from the wrong angle (where the illusion would look all wrong) would require being on the other side of the wall the window is in.

[darknite]

Won't play an illusionist due to the issue of needing common interpretation on how illusions work with any given DM.

[Segev]

Ooh, Xetheral, I'm totally stealing that. I'll have to think about edge cases for a while, but I think that manages to completely account for all of the behaviors of illusions, without breaking anything.It starts to beg questions of illumination. Is the illusion visible at the same luminosity regardless of ambient light? Can you see it in the dark? If not, how is that interacting with the "painted" photon? How do you determine when the ambient light matters and when it doesn't?

From a verisimilitude standpoint, as well, this starts to become more and more a matter of "your mind is being whammied into believing an obviously fake thing is what it pretends to be," as this kind of illusion can't help but look like, well, it's painted on. Which bothers me on a conceptual level.

I have no problem with it having light-and-shadow effects, or reflective effects, "outside" its AoE based on light bouncing off of it, because it really is hard-pressed to be all that abusive. And it's a lot easier to adjudicate: it looks like the object it's an image of looks under most circumstances.



EDIT: Yes, Segev, it's still only for a limited range of viewing positions, but much less limited than forced-perspective tricks. Make your arrangement of mirrors and lenses elaborate enough, and a 5' cube full of optics could look like a window opening on a vast landscape, from any angle through which a viewer could look through the window. To look from the wrong angle (where the illusion would look all wrong) would require being on the other side of the wall the window is in.
I've no real problem with that, though I'd definitely make the Illusionist make an Intelligence check (with proficiency if he has an appropriate Art or Performance skill) against DC 20 or higher for something so complex to look right. We're talking about something that is of limited, niche use under particular circumstances and difficult to pull off.

[tieren]

Yeah, unfortunately for the perspective tricks, it stops working long before you ahve to slow up to investigate. It won't look convincing enough to be problematic from five feet away, which is where you'd have to stop to investigate a hole. They can pretty much run up to it, slow for so little time that it won't be reflected in the mechanics of movement as their brains adjust from "I thought that was a hole" to "obviously something painted on the ground" and then continue on at full clip. You'd be served better by a false wall or rockfall or the like.

I disagree, I've literally almost stepped on some and had to take a stutter step when I saw it to check myself. When I looked I realized what it was, but for a moment it definitely gave me pause, which in the game I consider to be the investigation check.

How about this in your game: create the illusion of a 2 dimensional chalk drawing depicting a 3 dimensional hole in the floor, but put the illusion (of the chalk drawing) directly over an actual hole. The enemy approaches sees what is clearly a 2D chalk drawing which doesn't stand up to the perspective change as they approach so they ignore it and continue to chase down the hallway, falling into the pit trap.

Sure you could have just made the illusion of flooring covering the hole, but now how are those enemies going to react the next time they face you and see a 2D chalk drawing on the floor?

[Bobthewizard]

The problem with that is that as soon as you can create mirrors or lenses that behave like the real thing, you can completely bypass all of the size limits on illusions. You could arrange mirrors in a 5' cube in such a way that you produce an illusion of arbitrary size, and so make a castle from a cantrip.

This what I wouldn't allow as a DM.


The physics interpretation that I prefer is that illusions don't reflect or absorb photons at all. Instead, any photon passing through the illusion is "painted" by the illusion. When a "painted" photon hits a photosensitive cell of a creature who hasn't disbelieved the illusion, that creature sees the "painted" frequency rather than the frequency the cell is sensitive to. This makes the illusion visible even though photons aren't bouncing off it.

Interesting. But more science than I want in D&D.


As a simplicity hand wave, how much does it break things if visual illusions are just assumed to create "seemingly real but totally immaterial" objects/creatures (in that they visually act just like the real things until discovered to be false through Investigation)?

Nothing, except if they try mirror shenanigans, or optical illusion of a pit.


I allow it to block light. This rarely causes any problems, given the proliferation of Darkvision. And a torch or the like is easy enough to light. If the image is translucent due to making the investigation check, the. The viewer can see through it.

If you don’t let it block light, you wind up with really weird questions about light sources behind them.

If it doesn't block light, it looks funny. But if it does, it can get much larger range of darkness than the darkness spell. I think either choice is fine, just make sure everyone knows ahead of time.


Won't play an illusionist due to the issue of needing common interpretation on how illusions work with any given DM.

I don't play illusionists either. But I love minor illusion and silent image. They can be a lot of fun to both play and DM. At a table, it can be a quicker discussion than we've made it out to be here. Walls, lava, illusionary allies, auditory distractions, etc. are flexible and fun, if not overly powerful.


Here's a 1 hour discussion on how a DM can adjudicate illusions in 5e: https://youtu.be/6l51s0GFflY

Thanks. This was a helpful video. Essentially, he doesn't allow shadows or reflections, and uses passive perception about the same way I described.

Note: Edited the original post a little

[Chronos]

Right, I wouldn't allow it either. But a lot of people don't realize that "mirror shenanigans" start showing up the moment you have even a single flat plane mirror like the one in your bathroom or on your closet door. Which is why I would draw the line at no mirrors at all.

[Segev]

I disagree, I've literally almost stepped on some and had to take a stutter step when I saw it to check myself. When I looked I realized what it was, but for a moment it definitely gave me pause, which in the game I consider to be the investigation check.

How about this in your game: create the illusion of a 2 dimensional chalk drawing depicting a 3 dimensional hole in the floor, but put the illusion (of the chalk drawing) directly over an actual hole. The enemy approaches sees what is clearly a 2D chalk drawing which doesn't stand up to the perspective change as they approach so they ignore it and continue to chase down the hallway, falling into the pit trap.

Sure you could have just made the illusion of flooring covering the hole, but now how are those enemies going to react the next time they face you and see a 2D chalk drawing on the floor?I've never - sadly - seen one IRL, so I will have to defer to your anecdotal experience. I would not have issue with the faux perspective trap over the real trap for a double-bluff. Sounds clever.


Right, I wouldn't allow it either. But a lot of people don't realize that "mirror shenanigans" start showing up the moment you have even a single flat plane mirror like the one in your bathroom or on your closet door. Which is why I would draw the line at no mirrors at all.I don't see the problem, myself. "Making a castle out of a 5 ft. cube" is an exaggeration, to say the least, especially when you can only make it work through a five foot by five foot viewing port. I just don't see this breaking anything. And the efforts to prevent it all lead to much more annoying consequences if you want any consistency in how the spells work.

[Tanarii]

It starts to beg questions of illumination. Is the illusion visible at the same luminosity regardless of ambient light? Can you see it in the dark? If not, how is that interacting with the "painted" photon? How do you determine when the ambient light matters and when it doesn't?Main issue I have with it is that it should be instantly identifiable as an illusion any time the lighting from behind doesn't match the lighting from in front. No physical interaction or check should be needed in like 90% of the possible cases.

[Segev]

Main issue I have with it is that it should be instantly identifiable as an illusion any time the lighting from behind doesn't match the lighting from in front. No physical interaction or check should be needed in like 90% of the possible cases.

Right. This is one of several aspects that make this version, to me, seem like it'd stand out like a sore thumb, more obvious than the "this is going to break when stepped on" dirt at the cliff's edge in an old Hana Barbara cartoon. I just find "it's a not-real projection of the thing it's pretending to be into 3D space, and reacts to light and whatever other sensory impressions it works on the way a real thing would" to be the cleanest way to treat it. The Investigation check can find the flaws that arise from imperfect imagework, from misalignment, or from things passing through that would be hard to notice without looking for them.

In general, the Investigation check represents something in the world. You notice a real incongruity. The reason I think that they call out "interaction" as bypassing it is that it's just so gross an example of what Investigation might find that it essentially lowers the DC to 0. Which is also why I appreciate the "your attack roll is really an Investigation check" approach the OP suggested when fighting an illusion.

[Tanarii]

Conversely, I think of the Investigation check as successfully applying your powers of reason to overcome the force of the magic on your mind. So a physical interaction can't just make it go faint, it just lets you know it's an illusion, you still need to take an action an apply your deductive mental strength against it.

Otoh I also don't think interactions with light in physics terms are material. It just appears as best an object would in terms of ambient illumination from the casting illusionists perspective, but can't show a reflection any better than the casting illusionist can try and recreate.

I also think you can't just assume it can perfectly create anything the illusionist can think of. An fairly difficult Int check to create an illusionary double of someone they saw once in their life, a month ago, isn't out of order. And if you trying and create a horse without ever having seen one (and possibly designed by committee), you'll probably end up with a camel.

[Bobthewizard]

I've no real problem with that, though I'd definitely make the Illusionist make an Intelligence check (with proficiency if he has an appropriate Art or Performance skill) against DC 20 or higher for something so complex to look right. We're talking about something that is of limited, niche use under particular circumstances and difficult to pull off.

The spell description doesn't specify this so I'm pretty liberal in what they can come up with. OTOH I won't allow any mirrors or complex optical illusions.


In general, the Investigation check represents something in the world. You notice a real incongruity. The reason I think that they call out "interaction" as bypassing it is that it's just so gross an example of what Investigation might find that it essentially lowers the DC to 0. Which is also why I appreciate the "your attack roll is really an Investigation check" approach the OP suggested when fighting an illusion.

This check on an attack was meant for phantasmal force, which can react to the target's attack. For minor illusion, silent/major image, the illusion wouldn't react to the swing so if they attack I'd just allow an automatic hit and describe the attack going through and the illusion failing, without a role. I'll post my phantasmal force suggested rules in another post soon.


Conversely, I think of the Investigation check as successfully applying your powers of reason to overcome the force of the magic on your mind. So a physical interaction can't just make it go faint, it just lets you know it's an illusion, you still need to take an action an apply your deductive mental strength against it.

I think the spell descriptions are clear that interaction dispels the illusion, but as I said before, this interpretation is fine if it works at your table.


I also think you can't just assume it can perfectly create anything the illusionist can think of. An fairly difficult Int check to create an illusionary double of someone they saw once in their life, a month ago, isn't out of order. And if you trying and create a horse without ever having seen one (and possibly designed by committee), you'll probably end up with a camel.

The spell description just says the caster can create the illusion. I think not letting them make a horse unnecessarily nerfs the spells, even if they've only seen pictures. It would be hard to keep track of what the character has seen, so I let the caster make what they want, but if you are clear with your players, this is ok. It just seems harder to DM.

[Tanarii]

I think the spell descriptions are clear that interaction dispels the illusion, but as I said before, this interpretation is fine if it works at your table. Lets put it this way: if you allow physical interaction to make illusions go faint, they are incredibly weak, and you mave to make half a dozen new rulings, of which the three most important are::
1) What must interact? An individual themselves, a individual holding a object and poking it, an individual intentionally directing an object to interact (thrown rock, arrow), or any overlapping/intersecting object or creature.
2) When physical interaction happens, who does it go faint for? Everyone instantly (global), just those who perceive the interaction, just those who cause it to happen.
3) What if the illusion is of something that isn't easily interacted with? Fog or mist or leaves.

Classic use or not usable cases, depending on rulings on above:
A) Minor Image a 5ft box for cover (technically concealment), poke it with your finger, archer on the other side?. What happens?
B) Silent Image an illusionary fog cloud on you party, with archers outside. What happens?


The spell description just says the caster can create the illusion. I think not letting them make a horse unnecessarily nerfs the spells, even if they've only seen pictures. It would be hard to keep track of what the character has seen, so I let the caster make what they want, but if you are clear with your players, this is ok. It just seems harder to DM.Nothing says the caster can create a perfect illusion of anything they've ever known, seen even via picture, or can imagine. DM ruling is needed at some point so e players know the limits. OTOH I'm not suggesting it would look cartoonish or they require painting drawing skills to make realistic illusions. I assume at the very least, their training plus the magic make things look real-ish.

Edit: I also am not suggesting tracking what the player has or hasn't seen. That's ridiculous and a pain in the ass, as Druid Wildshape indicates. I let players judge that, I tell them my ruling, and let's them use that as their guideline when making their choices for casting spells. So far I haven't had to step in and say "no illusions of Giff-opotomus with AK-47s please". Or dwarf-mecha. Or tinker gnomes or kender. Things that don't even exist in my campaign.

[Chronos]

My ruling on physical interaction is that it applies for anyone who sees the illusion interacting (or more likely, not interacting) in a way that it should. See an arrow go right through a fog cloud? Yeah, that's normal for fog clouds. See an arrow go right through a brick wall? Something's obviously up. And it doesn't matter who shot the arrow.

[Bobthewizard]

Lets put it this way: if you allow physical interaction to make illusions go faint, they are incredibly weak, and you mave to make half a dozen new rulings, of which the three most important are::
1) What must interact? An individual themselves, a individual holding a object and poking it, an individual intentionally directing an object to interact (thrown rock, arrow), or any overlapping/intersecting object or creature.
2) When physical interaction happens, who does it go faint for? Everyone instantly (global), just those who perceive the interaction, just those who cause it to happen.
3) What if the illusion is of something that isn't easily interacted with? Fog or mist or leaves.

Classic use or not usable cases, depending on rulings on above:
A) Minor Image a 5ft box for cover (technically concealment), poke it with your finger, archer on the other side?. What happens?
B) Silent Image an illusionary fog cloud on you party, with archers outside. What happens?

Nothing says the caster can create a perfect illusion of anything they've ever known, seen even via picture, or can imagine. DM ruling is needed at some point so e players know the limits. OTOH I'm not suggesting it would look cartoonish or they require painting drawing skills to make realistic illusions. I assume at the very least, their training plus the magic make things look real-ish.

Edit: I also am not suggesting tracking what the player has or hasn't seen. That's ridiculous and a pain in the ass, as Druid Wildshape indicates. I let players judge that, I tell them my ruling, and let's them use that as their guideline when making their choices for casting spells. So far I haven't had to step in and say "no illusions of Giff-opotomus with AK-47s please". Or dwarf-mecha. Or tinker gnomes or kender. Things that don't even exist in my campaign.

I think your interpretations are fine. I'm not trying to debate right or wrong, just get ideas on how people handle them. Sorry if my last post seemed critical. I didn't mean for it to be. I enjoy the different points of view on this thread and am thankful for your contribution to that. Here are my answers to how I'd handle your questions, but they aren't necessarily right.

1-2. I think I'd use something along the lines of "interaction causes it to go faint for any who witness the interaction". It seems to make more sense to me, but I see your point too. So if I present the players with an illusion of a chest and they all see the fighter swing a sword through it, I'd have it go faint. It's reasonable to tell them that they see the sword go through the chest and let them decide if they want to investigate though before it goes faint. I read the spell the first way, but if you read it the second I could work with that as a player.
3. I think interaction won't work for fog or a ghost and it would still require a check.
A. if someone sees you shoot an arrow out of or through the box it would go faint for them. If you step out from behind a box to attack then go back behind it, it still works. If there is a small hole in the box for you to cast spells out of it still works. In my games, what I usually see is the caster hide in a box and cast cantrips while the enemy deals with the martial characters. My NPCs aren't going to waste time looking at a box when someone is swinging a sword at them. I just let it work.
B. Archers would need to make a check to see in, and your party members would each need to make a check to see at all. I would let the caster see through it automatically but there has been debate about that in this thread too. Ruling that the fog doesn't act like normal fog when the archers' arrows go through it would be reasonable too. Or counting the attack as their investigation check.

The intelligence check isn't unrealistic, it just seems more complicated than I want. My guess is in practice we are at about the same place. Your players will probably avoid trying to make things that require an INT check. I don't allow optical illusions or functioning mirrors. But otherwise, they can make anything reasonable. I'm not sure what a Giffopotomus is but if it's a lion head on a hippo body I'd allow that, since the character could imagine that even if they'd never seen one. I wouldn't allow an AK-47 though since it doesn't even exist in their world.

[Tanarii]

Not really trying to argue a way is right either, but rather point out what a DM / player need to think about and discuss for illusions before play, to avoid assumptions and conflicting ideas.

From past threads on the matter, your assumptions/rulings seem to be fairly par for the course. Any physically interaction that's percieved makes it go faint for those that perceive it is a fine way to rule, and illusionists will have to plan accordingly.

Illusionary Fog Clouds need some special thought IMO. In particular, a ruling to avoid is that standing in it will make it go faint, but merely shooting an arrow at it won't because fog billows. That makes it more powerful than an actual fog cloud, because the players can stand in it and see out freely, but attacking into it isn't easily done.

[ad_hoc]

One note is that in my experience a lot of people forget that the lower level illusions are stationary. This means that some things that could be created are going to look silly.

On the investigation/interacting -

IMO the investigation check is there to allow an illusion to be disbelieved if the character has no chance to physically interact with it. Eg. something out of reach or a sound.

Illusions are still plenty powerful, it's just most of the time they aren't a great choice to cast mid-combat. Rules working as intended.

Illusions definitely don't force the enemy creature to spend its action investigating it. Actions are too important, creatures aren't going to pause combat just to look at something.

(This goes for other things too like grapple. If the enemy creature still has something useful to do they're not going to abandon their actions to make grapple checks to free themselves.)

[Chronos]

I think I might rule that, even after a telling interaction (a sword passing right through what appears to be a person, for instance), it still doesn't go faint without an investigation check. This is because in a magical world, there can be other explanations: That person the sword went through might be a real ghost, for instance.

That said, even without the illusion going faint, a reveal like that is going to make it a lot less effective, at least against intelligent foes. Nobody's going to waste their time worrying about an "enemy" that swords pass right through when there are solid foes to deal with first.

[Reynaert]

But if it does, it can get much larger range of darkness than the darkness spell

That really rather depends on how the DM rules what happens when you cast darkness on a light source, doesn't it?
IMO, it would be very strange to rule that a sphere of darkness doesn't block light, but an illusion does.

(Hell, if you go RAW-lawyer on darkness, you could argue that a human can see something on the other side of a darkness spell, but an elf can't because they have darkvision and someone with darkvision specifically can't see through the darkness spell.)