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    Default How do you check vs illusions?

    Hey all,
    I'm currently playing an AT rogue and I picked minor illusion as a cantrip.
    Say I'm using it to hide, as a box, tree, etc.
    What sort of check would you use for enemies to notice it's an illusion?
    Would you consider allowing a check even when not being actively looking for things out of the ordinary?

    I thought about Perception vs 8+prof+int with bonuses or penalties according to the situation, but maybe there are better ways to handle it.

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    Default Re: How do you check vs illusions?

    As a DM, my take is that unless there's a reason to suspect otherwise, illusions seem real. So a creature would just assume the box is just a box until something gave it away. Like if the creature tried to touch the box.

    Once the creature has reason to suspect the illusion's true nature, it becomes an Intelligence (Investigation) check, typically against the spell save DC of the caster. If it the check succeeds, the illusion becomes "flat and transparent," allowing the creature to effectively see through it and you (no longer) hiding within it. If the check fails, the creature may still understand that it's seeing an illusion but is unable to see through it. You remain hidden.

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    Default Re: How do you check vs illusions?

    Honestly, this is something that you need to discuss with your DM. Illusions, more than almost anything else in the game, require clear communication between you two precisely because they are so subjective.

    Speaking as a DM, though, I'd probably default to Investigation instead of Perception to figure out if it's an illusion, as that's the check/stat most often used with illusions to disbelieve.

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    Default Re: How do you check vs illusions?

    ~30 years ago, my first PbP game used 2e rules. Back then, one "attempted to disbelieve" an illusion.

    Our party was attacked by a wyvern. The bard attempted to disbelieve it as an illusion. The very real wyvern snatched her in its jaws and started to fly off. The DM added "She believes!! She believes!!"

    There's even a Leslie Fish song based on that mechanism.

    https://www.youtube.com/watch?v=c6DNk68J5RE

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    Default Re: How do you check vs illusions?

    Quote Originally Posted by Kuulvheysoon View Post
    Honestly, this is something that you need to discuss with your DM. Illusions, more than almost anything else in the game, require clear communication between you two precisely because they are so subjective.
    This.

    If you're just looking for what our take would be, this is mine.

    First of all, there generally isn't a reason for an enemy to suspect an illusion is in play at all (unless maybe if the illusion is something ridiculous or out of place), so they would treat it as real. If something is revealed to be an illusion, most likely because someone physically interacted with it (e.g. shooting an arrow through it), then they'll be on their guard and may suspect an illusion.

    Once they suspect an illusion, there are generally two ways they can attempt to reveal an illusion. The first way is by interacting with it. The second way (and I want to stress that this is RAW, not just me making stuff up) is by making an Investigation check vs. your spell save DC. If they pass the check, then they can see through the illusion. If they don't pass the check, then they can't see through the illusion. This doesn't mean they won't still suspect an illusion, just that they aren't able to tell. It takes an action to make an ability check, so investigating will require them to burn their action.

    As you can see, the best way to investigate an illusion is just by poking or shooting it. Now, some stronger illusions are physically tangible, in which case that may not work. But for the simple illusions there's nothing better.

    The best way to leverage illusions, then, is to keep a foe guessing. Mix in real objects with your illusions. Conjure up an illusory wall to hide behind, then later (somehow) conjure up a real wall and watch the enemy waste their arrows shooting at it, thinking the real wall is actually an illusion. When a wall or a creature or an object suddenly appears, you want the enemy to be unsure if it's real or an illusion. If you're really good at the mind games, you can have them ignoring real dangers while pointlessly attacking illusions. One way to do this is to use illusion spells to mimic actual spells you have. Use Silent Image to create a wall of fire, or actually cast Wall of Fire. Done right, and they'll shrink back from the illusion, but confidently run into the real wall of fire. You can also use mundane items to fake magic using a bit of sleight of hand (especially using your invisible Mage Hand), so this can provide another source of a real effect that you can mimic with your illusions.

    Of course, that's only once they suspect that you're using illusions. Initially, they'll assume everything is real, so make sure to maximize your leverage of that opportunity. Also keep in mind some enemies may know who you are, and that you use illusions, so they would already be on the lookout for illusions.

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    Default Re: How do you check vs illusions?

    My rulings, based on the written rules, are:
    - anyone who observes a physical interaction reveals it is an illusion. But it doesn't go faint.
    - anyone who spends an action makes the investigation check vs spell DC to discern the illusion for what it is, and then it goes faint.

    Most creatures will treat something as real unless they have strong reason to suspect otherwise, right up until they see a physical interaction. But since all creatures (including the caster) have to make take an action to make it go faint, they're often still pretty good at providing concealment. Creatures will either often either ignore or otherwise adjust tactics instead.

    But yeah, definitely have a conversation with your DM if you're going to depend on illusions, especially in combat. For example, they may rule something like any physical interaction, including overlapping another object or the caster touching it, makes it go faint for everyone. That'll drastically alter how you use them.

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    Default Re: How do you check vs illusions?

    - anyone who spends an action makes the investigation check vs spell DC to discern the illusion for what it is, and then it goes faint.
    ...For just that one specific person, right? Everyone else still sees it (even though the investigator can tell them that it's an illusion, that won't let others see what's behind it). And probably the investigator telling them would give anyone else who investigated advantage on their check.

    Also keep in mind that this is a magical world, and there are other forms of magic in it. If a monster suddenly appears, and your arrows go right through it, that could mean that it's an illusion... but it could also mean that it's a ghost.
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    Default Re: How do you check vs illusions?

    Quote Originally Posted by Chronos View Post
    ...For just that one specific person, right? Everyone else still sees it (even though the investigator can tell them that it's an illusion, that won't let others see what's behind it). And probably the investigator telling them would give anyone else who investigated advantage on their check.
    Yes, yes, and no, not advantage. But they might take the time (action) to make the check themself, if the illusion hadn't been previously revealed to them by physical interaction.

    Also keep in mind that this is a magical world, and there are other forms of magic in it. If a monster suddenly appears, and your arrows go right through it, that could mean that it's an illusion... but it could also mean that it's a ghost.
    Agreed. But if they take the time (action) to make a check vs a non-illusion, that's an even bigger win.

    And to be clear, spending an action is usually a win for the illusion caster, even if it makes it go faint for the one creature. An action is a heavy cost in combat. A creature is only going to spend it if they really need to see through an illusion.

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    Default Re: How do you check vs illusions?

    Reiterating "talk to your DM."

    But, for me, I try to assume that creatures don't suspect illusions without reason, and that the "physical interaction" clause is an acknowledgement that the investigation DC drops to auto-success when something blatantly impossible happens. This is a more generous interpretation to illusions than many will give, admittedly: I do not assume that seeing something walk into or out of a cloud of fog immediately reveals the fog's illusory nature, for example. I would even grant some leeway if you took pains to make it look like whatever walked through the illusory wall was a ghost.

    I also assume that interaction with an illusion yields a freebie Investigation check, probably rolled secretly while the check the player thinks he's making makes little difference. The illusion reacts appropriately or the player's action fails in such a way that it doesn't reveal the illusion if the hidden freebie investigation check fails. But again, in certain cases, I'd grant discovering it for free, no roll, such as somebody trying to climb an illusory wall only to discover that their hands go right through it.

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    Default Re: How do you check vs illusions?

    Segev, how do you handle casters that hide behind an illusion they can see through because they poked it with a finger, but the opposition can't?

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    Default Re: How do you check vs illusions?

    Quote Originally Posted by Tanarii View Post
    Segev, how do you handle casters that hide behind an illusion they can see through because they poked it with a finger, but the opposition can't?
    Well, first off, I tend to assume casters know their own illusions are illusions and can see through them. But even if not, this scenario is one that comes up anyway without any house ruling, I think, because a caster CAN poke a finger through his illusion while nobody else is looking, if all else fails.

    And the rules cover it: the caster can see, and the others can't. It is a nice advantage for the caster.

    I suppose I forgot to mention that I also permit creatures to actively take an action to Investigate, if they suspect an illusion is present or even if they suspect "something is up." I feel the action cost is more than sufficient, coupled with the uncertainty of success.

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    Default Re: How do you check vs illusions?

    Quote Originally Posted by Corey View Post
    ~30 years ago, my first PbP game used 2e rules. Back then, one "attempted to disbelieve" an illusion.

    Our party was attacked by a wyvern. The bard attempted to disbelieve it as an illusion. The very real wyvern snatched her in its jaws and started to fly off. The DM added "She believes!! She believes!!"

    There's even a Leslie Fish song based on that mechanism.

    https://www.youtube.com/watch?v=c6DNk68J5RE


    I asked because pretty much 30ish years ago we started playing with this sort of treatment for illusions and I was wondering what's the current way of handling them. Talk to the DM is of course on the table, we played together for over 30 years, I have no problems discussing the thing.

    Thanks everyone for your insights on the matter.

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    Default Re: How do you check vs illusions?

    I'm not sure that disbelieving is an option in 5e. If you suspect something is an illusion, you have to either interact with it or make an Investigation check. I mean, try to disbelieve in something and then have someone poke it towards your eye; you'll still blink because on a subconscious level you still believe it to be real. Heck, you still get stuff like this with YouTube videos where you have something like a ball flying suddenly at very high speeds towards the camera, even though people know it's just a YouTube video.

    But if a player insists on disbelieving in something, perhaps the DM should allow it. "Your character now believes this to be an illusion," has a similar energy to, "You don't find any traps." "The rest of you watch as Bob walks like an idiot into the very real X."

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    Default Re: How do you check vs illusions?

    Quote Originally Posted by Segev View Post
    And the rules cover it: the caster can see, and the others can't. It is a nice advantage for the caster.
    Ah. I'd consider it abusing a loophole in the rules, under the "physical interaction makes things go faint" interpretation.

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    Default Re: How do you check vs illusions?

    Quote Originally Posted by Tanarii View Post
    Ah. I'd consider it abusing a loophole in the rules, under the "physical interaction makes things go faint" interpretation.
    Ah; I tend to forget that some people interpret that as the illusion itself changing, rather than it being faint because the observer has figured out for sure it is an illusion.

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    Default Re: How do you check vs illusions?

    Quote Originally Posted by Segev View Post
    Ah; I tend to forget that some people interpret that as the illusion itself changing, rather than it being faint because the observer has figured out for sure it is an illusion.
    Thats not relevant to my point, which is I consider the tactic of say putting a box over yourself then poking it so you can see through it abusing the intent of illusions.

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    Default Re: How do you check vs illusions?

    Quote Originally Posted by Tanarii View Post
    Thats not relevant to my point, which is I consider the tactic of say putting a box over yourself then poking it so you can see through it abusing the intent of illusions.
    Ah. I do not, personally. Certainly, if it were, I would view the rendering of illusions see-thru when "detected" to be a design mistake. But to each his own.

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    Default Re: How do you check vs illusions?

    Quote Originally Posted by Greywander View Post
    I'm not sure that disbelieving is an option in 5e. If you suspect something is an illusion, you have to either interact with it or make an Investigation check.
    I think the Investigation check is a reskinning of the Attempt to Disbelieve.

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    Default Re: How do you check vs illusions?

    Quote Originally Posted by Miele View Post
    Hey all,
    I'm currently playing an AT rogue and I picked minor illusion as a cantrip.
    Say I'm using it to hide, as a box, tree, etc.
    What sort of check would you use for enemies to notice it's an illusion?
    Would you consider allowing a check even when not being actively looking for things out of the ordinary?

    I thought about Perception vs 8+prof+int with bonuses or penalties according to the situation, but maybe there are better ways to handle it.
    For reference, the lead developer, Jeremy Crawford, basically just has the relevant check happen any time the creature would make any kind of Action that would interfere or investigate the surrounding scene.

    So that could mean a player could ask what's in the dark room, thinking he'll roll a Perception check, and the DM instead asks for an Investigation check. Or, to put it simply, any time something can be considered a "Search Action", it is, and then the DM decides what check to use as normal (not the player).

    That also means that a creature that isn't studying for something or pausing for enough time to take an Action (like if they were running) will never notice an illusion unless they run into it.

    Obviously, YMMV, but it definitely seems like a good foundation to build off of, and it's basically how I run my games. It sets a good expectation of how illusions work for both sides that allows PCs and NPCs to plan around them.
    Last edited by Man_Over_Game; 2021-09-20 at 10:33 AM.
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    Default Re: How do you check vs illusions?

    Quote Originally Posted by Miele View Post
    Hey all,
    I'm currently playing an AT rogue and I picked minor illusion as a cantrip.
    Say I'm using it to hide, as a box, tree, etc.
    What sort of check would you use for enemies to notice it's an illusion?
    Would you consider allowing a check even when not being actively looking for things out of the ordinary?

    I thought about Perception vs 8+prof+int with bonuses or penalties according to the situation, but maybe there are better ways to handle it.
    The spells who can be noticed as illusions by looking at them mention so in the spell's text.

    Minor Illusion says:

    If a creature uses its action to examine the sound or image, the creature can determine that it is an Illusion with a successful Intelligence (Investigation) check against your spell save DC.
    As such, I wouldn't consider allowing a check when not actively examining the image. Unless maybe if there is some other factors that "give it away".

    In any case, it would be Int with Investigation proficiency, because your senses/perception tells you that the illusion is the real deal, and you need to deduce it isn't with reasoning.

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    Default Re: How do you check vs illusions?

    Quote Originally Posted by Miele View Post
    Hey all,
    I'm currently playing an AT rogue and I picked minor illusion as a cantrip.
    Say I'm using it to hide, as a box, tree, etc.
    What sort of check would you use for enemies to notice it's an illusion?
    It costs an action unless the passive investigation score of the enemy is higher than the AT's spell save DC. I do the same for players if an enemy uses this against them.

    Quote Originally Posted by from the spell's text
    If a creature uses its action to examine the sound or image, the creature can determine that it is an illusion with a successful Intelligence (Investigation) check against your spell save DC. If a creature discerns the illusion for what it is, the illusion becomes faint to the creature.
    Perception doesn't enter into it.

    Quote Originally Posted by Man_Over_Game
    That also means that a creature that isn't studying for something or pausing for enough time to take an Action (like if they were running) will never notice an illusion unless they run into it.
    This. As a DM, I have to stay alert for this; when I can it makes the illusions work better, and when I forget I end up screwing it up for the player(s) .
    Last edited by KorvinStarmast; 2021-09-20 at 10:39 AM.
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    Default Re: How do you check vs illusions?

    Physically touching it reveals it no action needed

    They just aren't great in combat except for ambush situations.

    They are far more effective in exploration and social situations.

    Not everything is for combat.
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    Default Re: How do you check vs illusions?

    Quote Originally Posted by ad_hoc View Post
    Physically touching it reveals it no action needed

    They just aren't great in combat except for ambush situations.

    They are far more effective in exploration and social situations.

    Not everything is for combat.
    THe issue I take with it is that if you cast a combat-duration summoning spell of any sort, the DM will immediately have the enemies start reacting to the conjured monsters as if they were real (because they are), but often won't so much as waste a single creature's action on more than minimally "testing" the reality of the conjured monsters if he knows they're illusions. Even good DMs fall prey to this, just ignoring them as "unimportant" because, er, honest, the NPC creatures totally would not be concerned about the ogre that just jumped out of hiding and is bearing down on the monsters' egg clutch - the egg clutch the monsters are fighting the PCs to defend.

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    Default Re: How do you check vs illusions?

    Quote Originally Posted by Segev View Post
    THe issue I take with it is that if you cast a combat-duration summoning spell of any sort, the DM will immediately have the enemies start reacting to the conjured monsters as if they were real (because they are), but often won't so much as waste a single creature's action on more than minimally "testing" the reality of the conjured monsters if he knows they're illusions. Even good DMs fall prey to this, just ignoring them as "unimportant" because, er, honest, the NPC creatures totally would not be concerned about the ogre that just jumped out of hiding and is bearing down on the monsters' egg clutch - the egg clutch the monsters are fighting the PCs to defend.
    Well that's an issue in those DMs' DMing styles.

    Many DMs also do the reverse with illusions that have a more "concrete" effect, ex. making all NPCs act as if they didn't know becoming invisible or altering one's appearance were possible (even NPCs who are capable of doing it themselves or who have traditional enemies who can do it).

    Creating a world where the Evil Baron Badfellow won't waste a turn acting as if the illusory wolf who's charging at him is real, but also will accept that the Disguise Self'd warlock is the operative he put in charge of spying on the Thieves' Guild with little to no test.
    Last edited by Unoriginal; 2021-09-20 at 12:03 PM.

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    Default Re: How do you check vs illusions?

    Quote Originally Posted by Unoriginal View Post
    Well that's an issue in those DMs' DMing styles.

    Many DMs also do the reverse with illusions that have a more "concrete" effect, ex. making all NPCs act as if they didn't know becoming invisible or altering one's appearance were possible (even NPCs who are capable of doing it themselves or who have traditional enemies who can do it).

    Creating a world where the Evil Baron Badfellow won't waste a turn acting as if the illusory wolf who's charging at him is real, but also will accept that the Disguise Self'd warlock is the operative he put in charge of spying on the Thieves' Guild with little to no test.
    I've rarely seen DMs err on the side of buying an illusory disguise over assuming the disguise was the barest possible minimum to even allow an ATTEMPT at a Deception check.

    Fortunately, there's been a good example of disguise self working over long term in the game I'm in right now; we replaced the commander of a fort with another member of the same race (tabaxi) wearing a hat of disguise for a few days. The relative ability of the fort commander to remain - if not isolated, then at least aloof - and his reputation for harsh punishments for crossing him made it feasible.

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    Default Re: How do you check vs illusions?

    Quote Originally Posted by Miele View Post
    Hey all,
    I'm currently playing an AT rogue and I picked minor illusion as a cantrip.
    Say I'm using it to hide, as a box, tree, etc.
    What sort of check would you use for enemies to notice it's an illusion?
    Would you consider allowing a check even when not being actively looking for things out of the ordinary?

    I thought about Perception vs 8+prof+int with bonuses or penalties according to the situation, but maybe there are better ways to handle it.
    This would depend a lot on the situation. If they do not have a reason to suspect it's an illusion, they won't likely get to roll. For example: you made an extra box in a storehouse, or a tree in a forest and they did not see or hear you cast it.

    If you made a box appear during combat, I would give them a chance to suspect it's an illusion if they were observing you. Wizards would automatically know if it's on their spell list, but would still have to successfully disbelieve to see past it.

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