What would happen if an adventurer poured acid down a creatures’s mouth, while in a grapple? Say, the creature had the adventurer in its open mouth, who then pours acid down its throat. Just normal acid splash damage or any advice?
GM's are the only one who can say an action does anything more than the Rules as Written.
Rules as written, you can:
- ...draw a light weapon as a move action with a successful grapple check.
- ... make an attack with a(n)... light weapon against another character you are grappling. You take a -4 penalty on such attacks.
Drawing an Acid Flask and making a Ranged (or Melee, ask your GM) Touch Attack with a -4 penalty would allow you to deal Acid Flask damage to a creature. (Missing with such an attack is likely the result of the target diverting your hand to pour said concoction on the ground).
User blurry has a great, additional, point. As written, deploying a thrown weapon in this manner would incur improvised weapon penalty of -4 on the attack, representing the difficulty of uncorking and effectively pouring the acid onto the creature instead of yourself or the ground. (Imagine a waiter pouring water into a cup from a foot above it with his arm bound to his torso). Again a GM could waive this penalty, but now you're looking at a -8 to hit Touch AC. Still a reasonable endeavor, but one that could fail.
The rules don't cover pouring acid into a creature's open mouth and down its throat. The closest the rules come is this: "A character can carefully administer a potion to an unconscious creature as a full-round action, trickling the liquid down the creature’s throat" (Dungeon Master's Guide 229).
To be clear, there aren't any rules for feeding a conscious creature—willing or unwilling—anything, not booze, not potions, not Masters of the Wild-style infusions, and not alchemical creations like entire flasks of acid.
Of course, a DM is free to come with house rules, maybe extrapolating from the rules Variant: Damage to Specific Areas (DMG 27) or whatever, but this DM recommends against it: while it may be fine and cool the first few times the PCs pour acid down a grappled foe's throat, the tenth time foes pour acid down the PCs' throats and the PCs can't cast spells for a week (or whatever effect the DM's house rules have), the house rules probably won't seem as fine or as cool. Likewise, if the DM expands his house rules to include force-feeding potions, I'd expect every adventurer to wear masterwork potion belts filled with potions of painless death [necro] (Ghostwalk 56) (1st-level spell at caster level 1) (50 gp; 0.1 lbs.). To this player, being force-fed a potion seems a rather ignominious death for his heroic PC, and he'd feel better were his PC just the victim of a regular ol' coup de grace instead.
I feel that your question may be coming at the topic from a different direction than the D&D rules support, and it is one that I have seen cause D&D players some grief, and general misunderstanding vice-versa other rules-sets.
You seem to be describing an action, then looking for a way to get that result in the rules. The way I DM that seems to work best for me when running D&D, is to roll the attack and damage, then describe what happened.
So, for example, the player of a PC next to a monster might say, "I throw an acid flask at the Dire Rat." If they then roll and confirm a critical hit, then roll 6 or more hit points of damage, a possible description would be "You grab the dire rat's mouth and pour the acid down its throat. Poetic description here."
I derived this method from the fact that D&D has no "defined" method for specifying hit location, attack manoeuvre, or other comparable rules such as you'd find in GURPS. So, it seems reasonable that the intended sequence in D&D is, rather than deriving action from description then resolving result, we resolve result from action, then derive description from result. (Relevant historical note: I derived this from the opposite of what is described, having learned GURPS long after I learned D&D 3.5e)
Treat this as a Coup de Grace.
The amount of control required--open the flask, force the creature to open its mouth, pour the flask in without the victim turning their head away--is similar to what it would take to do something else lethal like stab them in the throat. In 3.5e, this means you (or someone else) must have them pinned. This would do damage as for an Acid Flask, automatically crit, and they would have to make a DC 10 Fortitude save or instantly die.
Balance-wise, if you can pour a potion directly into a creature's mouth, you can instantly kill them or otherwise mess with them however you want, so it should be as mechanically difficult as a Coup de Grace with any other weapon.
Now, if some huge monster was swallowing you whole, and you're a rogue carrying a lot of bottles of various poisons, then it makes sense that you could just open them and dump them into the monster's gullet on the way down. This wouldn't require any attacks or anything; the poisons would simply take effect.