What happens when Wild Shape/Polymorph runs out in a space that's too small?

Question

So in a 5e campaign I'm running, the party Druid has recently been doing some scouting of the local monster caves in spider form to avoid notice. I've also described how the goblins and kobolds who live in these caves use a variety of tunnels too small for Medium sized creatures in order to get around. Since the party doesn't have any halflings or gnomes, I didn't really think the party would ever get into those tunnels. But obviously if a goblin can fit, so can a spider, so there is the possibility that the Druid might try to scout them out.

But what if something goes wrong? What if the Spiderdruid were to get lost and run out of time in Wild Shape, or encounter a hungry lizard and drop to 0 Spiderhealth?

Is there anything in the rules to suggest what should happen when somebody tries to revert to their normal size in a space where they normally wouldn't fit?

With a space sized for a Small creature such as in my game, I might just rule that he's stuck unless he makes difficult Dexterity checks or waits to get his Wild Shape back. However, what if it was an even smaller space, such as a burrow or a pipe or a deep crack in the rock? Someplace where his human body literally won't fit?

If there aren't any rules for this, I'm open to suggestions on how it can best be handled.

Answer 1

Unfortunately, there are no rules for this circumstance, so if our itsy-bitsy-spider-druid runs out of Wild Shape while trying to climb up a downspout, we have to come up with a ruling, and there are three routes here:

1. The generous route is to let the druid stay shaped until they exit the downspout (or other space that can't fit their humanoid form at all), as the unshaping fails completely. If they are 0 HP while shaped in this case, then they stay at 0 HP, shaped, but unconscious, and have to make death saves just like any other non-stabilized unconscious PC.

2. The stringent route is to simply eject the druid from the too-small space; this smacks of video-game physics, but skews the balance away from the druid in that he can't be an itsy-bitsy-spider-druid hiding in a downspout for months or years. If ejecting him from the space is impossible (say, if someone sealed him into a small box while in itsy-bitsy-spider-form and FedEx'ed him), then you either choose a different route, or rule that ejecting the druid either passes through or destroys any obstacles that prevent ejection from the space.

3. The possibly-painful route is that the druid tries to unshape, and starts to enter human form in a space too small to contain a human, so he takes crush/constriction damage (and so does the object) until either the druid dies a horrible death (the continuous nature of this damage would make it impossible for him to make his death saves), or the object that was trying to contain the unshaping druid is destroyed, leaving a fuming druid amid a pile of shredded cardboard, a ripped downspout, or what-have-you. This would be analogous to the drowning rules, as it'd be incumbent on the player to avoid putting their character into such perilous situations, and considered only a fair consequence that the druid might die for screwing up that badly.

Of course, if the druid has another use of Wild Shape at hand, they can avoid all this hassle by burning that use of Wild Shape to avoid unshaping at all for the time being.

Answer 2

Covered in this question:

PHB 192

Squeezing into a Smaller Space

A creature can squeeze through a space that is large enough for a creature one size smaller than it. Thus, a Large creature can squeeze through a passage that's only 5 feet wide. While squeezing through a space, a creature must spend 1 extra foot for every foot it moves there, and it has disadvantage on attack rolls and Dexterity saving throws. Attack rolls against the creature have advantage while it's in the smaller space.

This covers your case of a Medium wizard going into Small kobold tunnels, at least, unless of course the tunnels are actually Tiny and the kobolds are themselves Squeezing through.

For cases not covered under rules - such as unshaping while in an inches-wide crevice for example, in this case - my preference is to consider realism, which is somewhat nonsensical to say in a fantasy game, but still: consider for example that Wild Shape transformation takes an action, and thus arguably several seconds, during which the form is functionally elastic. This would allow a conscious creature some small time to travel to a safe transformation space, or perhaps an unconscious one to be launched out of a crevice with harmful and/or comedic effect. Alternatively, as the DM you can also decide that such a situation would likely be fatal and warn the player ahead of time of that intent... Or not.