What happens when there's no space?

[SangoProduction]

Say you were under the effects of Diminish Person, or whatever it is.
You are then trapped in an impenetrable force cage (not the spell, a hypothetical object), just large enough for you to be squeezing at your current size.
Then the spell effect ends, and you're supposed to return to normal size.

What happens?

[Kurald Galain]

Adjudicating this kind of highly-unusual interactions is the GM's job, not something detailed in the rules.

I would rule that you die. Messily. Because forcecage is a 7th-level spell and it's entirely appropriate for 7th-level spells (or their equivalent "hypothetical objects") to kill people messily.

[Seto]

Agreed, it's a GM call. Personally, I would rule it similarly to Enlarge Person, where if there's insufficient space and the force cage is unbreakable, you just don't grow. Depending on whether I wanted to give the PCs a hard time, I might tell them they need to cast Enlarge person on the subject once they're out of the cage so that they go back to their normal size, or (more likely) I would let the effect end by itself as soon as possible.

[Fizban]

I'd say the most appropriate answer for most people is that depending on how much space there is, the risk ranges from suffocation to cave-in damage- this being a medium between "lol instant death" and "nothing happens" which has a mechanical backing. Alternatively you could go with 16, 18, or 20d6 (the final two DMG crushing X traps, or the round 20).

The question is clearly asking something that DnD hardly acknowledges, conservation of mass/volume, by explicitly establishing a setup where too much will have to fit in too little. Ruling that they die instantly doesn't fix the impossible volume compression problem. Ruling that the laws of reality refuse to allow this and the force effect will fail, creates a new exploit for taking down force effects. Attempting to calculate the forces involved in compressing the matter to the required size in case of a simply metal enclosure likely invokes ridiculous values. Saying the universe clips you out like a teleportation spell allows another possible set of abuses, and so on.

But hey, DnD doesn't care. A Crushing Room trap can explicitly smash everything down to a 1" width, and it will still only deal the given damage. There is no given damage for "size reduction spell wearing off within a confined space," there is no rule that says you can't exist in a space smaller than yourself, guess that means nothing happens eh?

The spell wears off, you expand as far as is possible, fail to break the container with a strength check, and then if and when there is room to do so you finish expanding. Does this mean you're in some weird "limbo" state where the magic has worn off and yet you're still apparently affected by the magic indefinitely? Sure, and it's not like there aren't other spells that do that and even "extraordinary" abilities that flat out defy real-world physics. The standard expectation of the game is that a spell wearing off does not have inherent negative effects, and there is no mechanical or logical reason to impose them- it's pure personal preference.

[Psyren]

All the growth spells (Enlarge Person, Expansion, Animal Growth, Righteous Might etc.) include a "constrained without harm" clause if you fail to break your enclosure. I would apply that to a size-reducing spell ending - you don't get crushed but you can't move either until you somehow shrink again or someone gets you out another way.

[Elves]

From enlarge person:

"If insufficient room is available for the desired growth, the creature attains the maximum possible size and may make a Strength check (using its increased Strength) to burst any enclosures in the process. If it fails, it is constrained without harm by the materials enclosing it— the spell cannot be used to crush a creature by increasing its size."

Would make sense to apply this to reduce person as well.

[Krowmeat]

Now, so far this thread's had some sensible replies, but there was an old minutephysics video that suggested that an unstoppable force and an immovable object would simply phase through each other. If forcecage is truly unbreakable by any means and there's no listed way to stop a shrunken creature's return to normal size, then the creature would just phase through the tiny cage and regain their freedom, albeit with an unbreakable magic cage somewhere in their skeleton.

Now, this does break physics, but considering we're dealing with a magically shrunken individual and a cage that may or may not be denser than an object physically can be it's safe to say there's not many realistic ways to play this.

[icefractal]

"You only grow as large as possible" would work, and is probably the best from a "keeping interactions separate" perspective.

But how we ruled a similar situation recently (if a medium creature polymorphs into a huge creature with swallow whole, swallows a large creature, then the spell ends) was that both creatures would take damage at an equal rate until one died. So that's another option - you vs the hp of the container.

Another hypothetical one that's interesting is - with Polymorph any Object, or with the Transform Object talent from SoP, you can turn an object into a creature. If you turned a colossal block of stone into a mouse, had it hide inside a fairly small room, and then the spell wears off ... well at the very least, that room is filled with stone, if not entirely destroyed.


Huh, now that I think of it, the "expands only to fit the available space" has some fun interactions of its own:
A) ... And then it stops expanding forever. You can permanently shrink things or people by temporarily putting them in a strong container.
B) ... And when the container is opened or broken it will resume expanding. You can stick things in a can to keep them shrunken until you need them, then pop it open and boom, instant wagon. I guess this is how Capsule Corp operates.