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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)

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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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Polymorph changes the target's insides...

(Not a RAW answer...)

If Polymorph did not modify the contents of a target's stomach, intestines, bladder, etc., then the spell would be deadly to the target in a number of scenarios.

  1. Stomach contents are now too large for target creature's new stomach size.
  2. Stomach contents are now poisonous to target creature's new form.
  3. Bladder is too small for bladder contents
  4. Intestine contents are now too large to pass or worse too large for the new form's intestines
  5. Volume of air in the lungs changes too fast to equalize air pressure...
  6. Sinus cavity content volumes are mismatched...
  7. Uterus / oviduct contents no longer align with new species *
  8. bacteria/viruses that are deadly to new species but not to the original, if your interpretation of D&D worlds includes germ theory...
  9. Parasites, internal or surface...

In each of these cases, the target creature could face extreme discomfort, internal injuries, and death. Example 1 is your PC, as food, now no longer able to fit in the stomach of your target creature.

Since the rules don't include exception cases for live stomach contents vs. non-living, but they do seem to allow for the contents to transition without harm, one could assume this means your PC either "shrinks to fit" or becomes some food that is safe to eat and roughly the same proportional volume as in the original form of your target.

This does raise the interesting question of unborn babies / eggs / etc., but I suppose they would polymorph with the parent.

So what happens to your PC?

I would suggest a saving throw for your poor, swallowed, PC. On a save, the spell fizzles. On a fail, the spell fires and things get nasty for all involved. Perhaps you don't change, but the creature does. How much damage do you take from being inside a creature as it instantly shrinks and then explodes around you? Or perhaps you shrink to fit the new stomach size?

An alternative answer would be that you are "safely" expelled by the spell as the creature shrinks down to the new form. Along with any other intestinal/stomach contents that don't fit. And any parasites, etc. that don't fit on/in the new host form. And any bladder overflow. and...

Suddenly the entire idea of casting Polymorph is pretty horrifying.

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  • \$\begingroup\$ I feel that the player if they failed should meld into the creature that swallowed them, as they are being held \$\endgroup\$ – Fering Jun 15 at 20:55

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