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If you get swallowed by a Tarrasque, your only way of getting out is to deal it enough damage to force it to regurgitate you (60 damage or more on a single turn).

There's no mention of the AC when attacking from the inside of the monster. I'd presume that means that the AC is the same at the AC on the outside of it, i.e. 25, which is stated as "Natural Armour". This doesn't seem right though, because there is no natural armour on the inside of a creature, at least nothing as tough as the creature's carapace. The inside of creatures tend to be soft and vulnerable.

I guess this is just a juxtaposition of game mechanics vs game flavour, but I'm pretty sure that if my fighter somehow swallowed a creature, it sure as heck wouldn't be using my AC of 26 for the attack rolls!

TLDR:

If you attack a Tarrasque while swallowed, what AC do you need to beat to hit it?

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    \$\begingroup\$ "The beast's hide is too thick to be pierced from the outside. I must cut through it from the inside." \$\endgroup\$ – Vigil May 17 at 10:48
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    \$\begingroup\$ @Vigil: I understood that reference :) \$\endgroup\$ – V2Blast May 17 at 11:12
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Stat-block-as-written, as you say you use the creature's AC of 25 no matter what angle you're coming at it from. There's no general "creatures that are swallowed" rules to follow somewhere else; what you see in the stat block is all the rules you get.

If you're looking for a creative/flavor justification for why it's still 25 from within, I'm sure a smart DM could come up with one. Maybe it's really hard to figure out where exactly to hit while inside.

If a DM wanted to tweak the stat block of the Tarrasque they were running in their campaign such that it had a lower AC from the inside, that seems like a perfectly reasonable thing to do to me. I tend to think it wouldn't affect the monster's challenge rating in a meaningful way, though to be fair it's not like I've done any playtesting of such a change.

Really it comes down to if you're playing a kind of game where the DM is happy to bend or break rules according to "what makes sense" and the story you're all telling, or if you're playing a kind of game where everybody is expecting consistency and numbers to be basically as written. This is not really any different than other situational modifiers that some DMs might sprinkle in freely regardless of the rules, while others wouldn't.

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  • \$\begingroup\$ I disagree with the last part. In some cases (the right resistances) it is safer to stay inside and kill it from within. Lowering the AC for swallowed creatures can open loopholes. \$\endgroup\$ – Mindwin May 17 at 13:17
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    \$\begingroup\$ @Mindwin The Tarrasque in particular outputs 56 (16d6) acid damage per round and it's Hit Points are 33d20 + 330. Even a creature with acid resistance would only have 6 rounds at most to do ~670 HP of damage to the Tarrasque, attacking at disadvantage. \$\endgroup\$ – illustro May 17 at 13:26
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    \$\begingroup\$ @illustro not to mention that the Tarrasque has a 50% chance to regurgitate the creature after each turn if more than 60 damage is dealt. \$\endgroup\$ – David Coffron May 17 at 14:21
  • \$\begingroup\$ @Mindwin: I've added a bit of a disclaimer that I haven't done any playtesting of such a change. (And I added another paragraph at the end about how it's really just another case of general DM discretion.) \$\endgroup\$ – Peter Cooper Jr. May 17 at 15:22
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We need to figure out how the AC was calculated to come up with an answer.

The DMG rules for Creating a Monster (under Creating a Monster Stat Block > Step 7. Armor Class > Determine an Appropriate AC) state:

A monster that doesn’t wear armor might have natural armor, in which case it has an AC equal to 10 + its Dexterity modifier + its natural armor bonus. A monster with a thick hide generally has a natural armor bonus of +1 to +3. The bonus can be higher if the creature is exceptionally well armored. A gorgon, for example, is covered in steely plates and has a natural armor bonus of +9.

The natural armor bonus in this description clearly comes from the hide of the creature (since it is referenced exclusively in that context). Creatures don't have "hide" on the inside of them. Therefore you don't need to overcome that natural armor if you are attacking it from the inside.

Thus we conclude that the Armor Class on the inside of a creature with natural armor is different to it's AC attacking it from the outside.

In the case of the Tarrasque it's natural armor bonus is +15 (10 + 0 [Dex] + 15 = 25).

As a result, the AC for hitting the Tarrasque from the inside should be 10 instead of 25.

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  • \$\begingroup\$ Comments are not for extended discussion; this conversation has been moved to chat. \$\endgroup\$ – mxyzplk May 17 at 17:53

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