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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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They should be safely expelled.

When a creature is polymorphed, it gains the stat block of the form it was polymorphed into, as is stated by the Polymorph spell:

The target's game Statistics, including mental Ability Scores, are replaced by the Statistics of the chosen beast.

As a result, it loses the special ability that allows it to swallow a character - for instance, a Giant Frog polymorphed into a Horse would lose its Swallow ability. Therefore, the ability would end, since the creature no longer has the ability to sustain it, and the PC would no longer be affected by that ability, safely expelling them from the creature without either of them taking damage.

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    \$\begingroup\$ As a DM, I think this is the best ruling and it is the one I would make. With that said, I think saying that this is the result under RAW is a bit of a strain and I agree with Miniman that it is left to the DM. \$\endgroup\$ Aug 3, 2021 at 20:31
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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, 2019 at 20:55
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Such special cases are not covered specifically in the rules. (RAW lacks specific language for it).

So, it is up to the DM.

One "simple common sense" approach that I've seen applied by several DMs (as the "polymorph back to my true size while inside a strong enemy" is seemingly a surprisingly common rules exploit, the same as "drown it with Create Water", or "blind it with Light", and lots of others) goes like this:

Think if you cooked some raw muffin paste in a small cardboard in the oven. As the muffin paste volume increases to become muffin-shaped, the end result could be that the box rips apart a bit or even a lot (as in: some damage is dealt to the box). But the muffin itself would also definitely end up coming out of if really weirdly shaped (as in: some damage is dealt to the muffin too).

The player in the original question basically wants to come out as an intact pristine muffin, with a completely torn to shreds cardboard box. Letting him do that would just open the door to completely ruin campaign game balance, so obviously the DM has to make some ruling somewhere.

However, despite "up to the DM" being a valid RAW answer (per the DMG "master of rules" section and PHB p. 6) inside the Enlarge/Reduce, Wild Shape, and Polymorph RAW powers' wordings, the RAW rules show hints that no harm comes from using them.

  • Becoming bigger only goes up to the available room, not leads to crushing damage.

  • Equipment appears / disappears merging seamlessly and harmlessly.

  • Mouth and stomach content also seem to appropriately change harmlessly. Otherwise, with the mere act of turning into a fly, whatever little water was in your mouth as a humanoid PC, would then be several times the size of you as a fly, most of it quickly gushing out of your mouth in all ways possible: outside of your mouth but also right inside your lungs, too. Thus, you'd end up as a rather drowned fly.
    Or, whatever food was in your humanoid stomach, even an almost unchewed and quickly gulped down hamburger bite you just took the very round before, would end up being a huge amount of food compared to the size of fly, and thus would simply explode the fly's stomach (and the rest of the fly too). That water or that food are not "objects" (and thus not "gear") as defined by the rules, because objects have to be discrete things. Thus, there is a precedent for how to deal with "non-gear" matter on or inside a creature: No harm seem to be hinted to happens from such things.

In the same vein as the above "RAW hints", no harm should result from potential polymorphing hijinks when one creature is inside another.

If the DM is feeling permissive and wants the Rule of Cool and/or the Rule of Fun to apply in a stronger way than mere RAW indications seem to hint at, he should keep in mind the RAW "Damage Caps on Spells" in the DMG. Don't let a (for example) Level 4, utility spell becomes an insta-kill-no-save attack spell!

And don't give to a utility spell the same damage as a pure attack spell: allow for the fact that those damage caps are for spells that mainly only do damage, not for spells that you allow to end up doing damage plus utility.

Finally, remember to require a Saving Throw somewhere. The number of "no to hit no saves" spells are extremely limited in the game, for good reasons.

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    \$\begingroup\$ Why did you post a second very similar answer instead of editing your first answer? \$\endgroup\$ Aug 3, 2021 at 20:19
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    \$\begingroup\$ Your use of "RAW" doesn't support your argument. You might as well cut those instances from your text to focus on what you bring to the table instead of this weird appeal. \$\endgroup\$
    – Akixkisu
    Aug 3, 2021 at 20:19
  • \$\begingroup\$ Right, so your first sentence is using RAW in a non-conventional way. To say "RAW, such cases aren't covered by the rules" means "There is a written rule that explicitly states that such cases are not covered by the rules." But this isn't the case, there is no rule that says that, so I'm not sure what you mean by RAW when you aren't referring to any written rule. \$\endgroup\$ Aug 3, 2021 at 20:25
  • \$\begingroup\$ Fixed a bunch of spelling errors, tightened the prose slightly; please review to make sure the edit captures your intended meaning. \$\endgroup\$ Aug 3, 2021 at 21:07
  • \$\begingroup\$ Thanks Korvin! Did not see any Review button but you seme to av3e done a good job. \$\endgroup\$
    – Pat
    Aug 10, 2021 at 0:12

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