[RPG] What happens when a creature that has swallowed another creature is polymorphed


Twice this week, I have faced a situation where Polymorph was going to be cast on a creature that had swallowed a PC. As a DM, I ruled that if the polymorphed form was smaller, the swallowed PC would burst out, causing damage to the creature. As a player, my DM ruled that the spell would simply fail.

Are there any rules on what should happen in this scenario? The texts of Polymorph and the swallow ability of various monsters don't seem to address this situation.

(Related, but for 4e, and Related, but in the other direction)

Best Answer

There are no rules for this, so it's always going to be up to the DM.

Polymorph can be used on "a creature you can see", with no further restrictions. It (for all intents and purposes) automatically fails against shapechangers and creatures with 0 hit points, but that's as far as it specifies. There's one clause in Polymorph which could be argued to apply here:

The target’s gear melds into the new form. The creature can’t activate, use, wield, or otherwise benefit from any of its equipment.

So if your DM considers a swallowed creature to be "gear" (unlikely), it would meld into the new form. Otherwise, Polymorph has no rules for this situation.

As you've noted, the rules within each creature's statblock have nothing that covers this situation either. It's noteworthy that, while each of them can only swallow creatures of a certain size, there's nothing that says what happens if a creature they've swallowed grows beyond that size. So, as a DM, I could certainly see an argument that when a creature you've swallowed becomes too big for you, the swallow ends messily.

Another point to consider about the rules for swallow within each creature's statblock are just that - within each creature's statblock. There are no global rules for swallow. Well, so, what?

The target’s game statistics, including mental ability scores, are replaced by the statistics of the chosen beast.

This means that, once Polymorph is cast, the rules that allowed the creature to have another creature inside it have ceased to apply to that creature. What does that mean? The only sensible way to deal with this paradox is to accept that the rules just don't cover this and the DM is going to have to decide what happens.

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