What happens if you polymorph something like an elephant into a frog, and then place a forcecage around said frog with the polymorph spell ending before the forcecage does?
Does the elephant just appear outside the forcecage or does the elephant turn into a compact cube of meat?
Forcecage, page 243 PHB
A prison in the shape of a box can be up to 10 feet on
a side, creating a solid barrier that prevents any matter
from passing throllgh it and blocking any spells cast into
or out from the area.When you cast the spell, any creature that is completely inside the
cage's area is trapped. Creatures only partially within the area, or
those too large to fit Inside the area, are pushed away from the
center of the area until they are completely outside the area.A creature inside the cage can't leave It by nonmagical means.
In this scenario, this is a Tiny sized force box around the frog, so this will end up resulting in a Huge sized elephant in a Tiny area (up to 10 feet on a side) that no matter can escape from.
The RAW vagueness comes from the second and third paragraph. Is the second sentence of the second paragraph an extension of the 1st sentence, as in it only applies upon the initial casting?
Also, would the Polymorph spell ending be a "magical means" to leave the forcecage, applying to "those too large to fit Inside the area, are pushed away from the center of the area until they are completely outside the area."?
Best Answer
Rules as Written are Unclear
The Force Cage spell states (PHB, pp243)
And then separately:
Given that these are are two separate sentences, the "too large to fit" clause may or may not be directly tied to the "when you cast the spell" clause. If it applies to both at time of casting as well as during the life of the spell then the creature is shunted (if it is two sizes larger than the cage - see squeezing rule.) If it only applies to the cage creation, then it raises the issue of a creature being in too small a space (see below.)
If those two sentences are supposed to be directly related, it changes the interpretation and then allows for use of Force Cage to autokill something. There are no real rules for this, and this question on polymorph may be related with regard to too small a space. See the Concerns section below for more information.
Squeezing into a smaller space
On the chance that the polymorphed creature is just one size smaller then the creature itself, the Squeezing rules would come into play (PHB, pp192)
Concerns
There are two concerns that you should consider when determining use of this at your table:
If you choose to read the Forcecage text as not applicable post-casting, then you have created a neat effect to instantly kill a creature. This could become very powerful if you have a group that is able to do this. However, this is also using a 7th level spell slot (along with a 3rd) - and may not be as unbalanced as it seems. This is similar, but not identical, to the idea of polymorphing into a tiny creature to be eaten or go into a larger creature and then end the polymorph so you "burst out" and kill the creature. There are no RAW rules around this and allowing tactics like this potentially creates a slippery slope (although not as great when you consider the 7th level spell cost for Force Cage.)
If you choose to go with the interpretation of being shunted out, you are risking devaluing the Forcecage. The odds of this happening are small as it is unlikely to be used against the party by an enemy.