I discussed this a few months ago on google+, but it seems pertinent to go into some more detail here.
The ability of large monsters to swallow their prey whole is a time honored part of the game. But as best I can tell, there’s not actually a lot of detail on how it’s supposed to work. The AD&D DMG doesn’t mention it in the index, and in the monster manual it tends to simply state that the swallowing happens. Such as this entry for the T-Rex:
“This monster will pursue and eat nearly anything, engulfing man-sized creatures whole on a rolle of 18 or better.”
That’s the end of the entry. A perfectly reasonable interpretation would be that a character who is swallowed whole is now dead. But that feels pretty cheap. A more interesting idea is that the character is now in a state of limbo. If the remaining party members slay the beast then their friend can be saved; if the monster gets away, then their friend is dead.
But shitty movie cliches and lenient referees have convinced most players that being swallowed whole is somehow actually beneficial to them. After all, their sword can’t exactly miss when they’re entirely surrounded by the soft and vulnerable flesh of a monsters insides, right?
From the player’s perspective,though, it’s not an entirely unreasonable request to want to continue fighting so long as they’re still alive and have a reasonable expectation of being conscious. So I think we have to give them the opportunity. But the mechanics of the situation need to represent the fact that the players are at a severe disadvantage. If they actually manage to free themselves, then it should be impressive not because the imagery is so cool (it isn’t. It’s cliche and boring). It should be impressive because the odds of success seemed so incredibly remote.
SO, Miscreated Creatures will include a set of “Standard Swallowed Whole” rules in its appendix. Monsters will either swallow whole according to the Standard Rules, or they will list their deviation from such. I haven’t 100% settled on how the Standard Rules should look yet, but this is where I’m sitting currently:
When a character is swallowed whole, they must succeed on a save versus Paralyzation to determine if they keep hold of their weapon. Bludgeoning weapons like hammers or fists are useless. It’s impossible to get sufficient momentum with such weapons to do any good. Whips fail for the same reason. Most ranged weapons, such as bows, are similarly useless. Guns work, but cannot be reloaded once fired. Any weapon that is too long (a spear, a pike, a musket) wont’ be able to be swallowed correctly, and will thus be broken in half when swallowed. Characters who make their save will be able to use any slicing or piercing weapons they can hold on to.
Attacks made while swallowed whole automatically hit, unless a 1 or 2 is rolled, in which case the character loses their weapon and can’t get it back. Otherwise, roll damage. The creatures takes only half of the damage rolled. Creatures who swallow live prey do not have delicate stomach tissue.
The acids and lack of breathable air within a creatures stomach will be suffocating for any character trapped within. After 1 + Constitution Modifier rounds, a character must make a save versus Poison each round in order to remain conscious. Swallowed characters take 10% of their total hit points as damage each adventuring turn.
Using these rules, a swallowed character’s ability to resist will likely end within only a few rounds. Even a high level halfling with 18 CON has a 10% chance to succumb each round after the 4. But their death will be slow, allowing their companions time to rescue them by hunting down and slaying the creature even if it flees.
These rules are a little over complicated perhaps. Much as I like simple rules, I have a habit of writing rules the way Pathfinder taught me to. TL;DR:
Save v. Para to determine if you can hold onto your weapon. Awkward weapons don’t work. 1-2 attack roll drops weapon, all others are hits that deal 1/2 damage. Save v. Poison to stay awake after 1 + Con Modifier rounds. Take 10% damage each adventuring turn.
Of course, some monsters will have steel bellies that can’t be harmed, or their bellies will be full of fire that kills you very quickly. Unpredictability is the name of the game. But I think this forms a good basis from which to adjudicate monsters that swallow characters whole.