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I'm running a tier 4 game and ran into an interesting situation.

1 of my players had been swallowed by a gargantuan sized homebrew creature. On his turn he used the Shapechange spell to become a Dragon Turtle (also gargantuan) in size.

What do the rules say should happen in this situation? Does he burst out thereby killing the monster? Is he somehow shunted out? Does the spell fail? or does something else happen?

Akixkisu
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Ryan Chaplin
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1 Answers1

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The rules are intentionally silent

What do the rules say should happen in this situation?

The rules do not describe this situation. Instead, the rules imply the DM will decide:

"... the rules can't cover everything. When you encounter something that the rules don't cover or if you're unsure how to interpret a rule, the DM decides how to proceed"

Tasha's Cauldron of Everything page 4, "The DM Adjudicates the Rules"

It's the DM's job to make decisions and adjudications during the game. The lead game developer supports this approach, using a very similar example:

What happens when someone swallows a druid in a Tiny form? <...> The rules are intentionally silent on these corner cases, leaving adjudication to DMs.

This is a part of the larger principle "rulings over rules" from earlier editions, which 5e tries to resurrect. See Where have the designers of 5e said that they favored rulings over rules in this edition?

So what are the options?

As a DM, you have a few options. Choose one depending on the player's intent, your own play style, common sense, and table conventions:

  1. Shapechanging into a gargantuan creature kills the swallower instantly. Can be probably abused, so use it with care. Kind of gross. Can also be used against PCs as well (ask the players, if they are okay with that).
  2. The same, but the caster also takes damage. A trade off for an insta-kill. Still gross.
  3. A caster can't Shapechange if there is no space. The magic doesn't work like that. A polite way of saying "no". Fair but harms creativity.
  4. The middle way — the swallower throws up, probably suffer damage in the process. Probably the optimal one, kind of boring.

Choose one and stick to it. Consistency is the key — players doing a same thing should get the same (expected) results. More info: What is Player Agency and what is it good for?

We play games to have fun, so players' (and the DM's) fun is the first priority. This makes the "Rule of Cool" completely legit. See the D&D Adventurers League: DMS for details:

Run a fun game! As the DM, you help guide the players through the adventure, and facilitate the story coming alive. Make sure you follow the golden rule: Make decisions and adjudications during the game that enhance the fun when possible.

Also on-topic:

Inspirations:

enkryptor
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    Matthew Mercer is a sketchy source for a supported answer (albeit as a DM, he has many positive approaches to the game) – KorvinStarmast Dec 09 '20 at 14:03
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    I recommend an addition to option 1: The spellcaster takes damage. Whatever force their expanding form exerts on the interior of the creature will also be exerted on the outside of the player. – Engineer Toast Dec 09 '20 at 18:56
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    A slightly variant on #2 I've used is that the swallower takes damage at a 2:1 rate (since ones inside is generally less durable than the outside) until one side or the other dies. Makes it a risky but not impossible option - an even split makes this more of a suicide tactic against most things big enough to swallow someone. – Errorsatz Dec 09 '20 at 19:53
  • So, what happens if the druids tries to shapechange in a building that doesn't accommodate the new size? And if that's covered by the rules, can that be extended from a building/hard structure to soft fleshy containers? – ptyx Dec 09 '20 at 21:13
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    @ptyx that is not covered by the rules – enkryptor Dec 09 '20 at 21:38
  • Rule of Cool: >>Pop!<< or 1/2 creature's HP as damage + regurgitation. Teach 'em to swallow a druid! – Senmurv Dec 09 '20 at 21:57