A door opens to present a 20' diameter circular room. The walls of the room are smooth and featureless greenish-gray stone, save for the wall directly across from the door, which features a 5'tall face in relief, seemingly carved out of a projection of the living stone. Light glints golden-yellow off of something covering the lips...
Upon closer inspection, the face projects about three feet out from the wall. It is humanoid of sort, certainly of masculine appearance, though thick of features and not of any particular race. The face has large close-lidded eyes, a large bulbous nose, wide elephantine ears with pendulous lobes, thick lips, and a jutting chin. The most remarkable thing about the face is its lips, which appear to be sewn shut with a thick string of gold!
That is, they seem the most remarkable thing, until the lids of the eyes flip wide open (which they do when any being approaches within 10 feet). The eyes are brilliant flashing green, like star emeralds, the pupils being the star of the gem. The eyes go wide when they see anyone who has a cutting weapon, and then the face writhes in pain as the lips purse out toward that person, the eyes pleading.
To all save the very dim, it is obvious the face wishes them to cut the golden string. If the adventurers argue among each other against cutting the string, or turn to leave, the eyes of the face tear up, and it begins to weep and sob and snort sadly. If the adventurers leave, as they pass through the door they hear a horrible burbling whine come through the sewn lips, with a muttered "..hhhhpppllllzzze?"
If the adventurers are so foolish as to actually cut the golden string, disaster, naturally, awaits them. Upon being cut, the string falls away immediately and the mouth flies open, the face giving off a great shout of joy. Then, a huge, 30' long black tongue darts out of the mouth and attacks as a 10 HD monster! First, it attacks the adventurer who most strongly fought against cutting its bonds. The tongue attacks by grabbing at the target; a successful hit deals no damage, but will grapple the target with an inhuman and irresistible strength, and on the next round will drag the target into the mouth completely.
Any target dragged into the mouth is stuck in the face's extra-dimensional gullet, and suffers 3d10 points of damage per round from the terrible acids. When an adventurer reaches 0 hit points, his body and most of his equipment are completely dissolved beyond recovery. Any magic items or gems, however, remain, and will be spit out as opportunity permits.
If the tongue is attacked, it is AC 2, immune to non-magical weapons, suffers no damage from bludgeoning attacks, and has 20 hit points. If the tongue is destroyed it falls into a pile of black ichor, the face puckers up angrily, and spits out a mist of black demon blood poison in the next round. Anyone in the room must make a saving throw against Poison or die. The tongue regenerates at a rate of 1 hp per turn.
The face can be attacked; only two can melee with the face directly at any time. Any such attackers immediately become targets of the tongue by default, the tongue attacks them with a +4 bonus, and if hit, they are dragged in immediately in the same round. The face has an AC 0, is immune to non-magical weapons, and has 50 hit point (separate and distinct from the tongue). If the face is slain, it crumbles in on itself and the tongue, if remaining, falls into a pile of black ichor. If merely wounded, the face regenerates at a rate of 1 hit point per turn (again, separate from the hit points of the tongue).
The face will continue to attack until it has consumed all the adventurers, save the one who cut it free... provided said adventurer has not attacked it in the meantime, in which case it will show no mercy. However, if the adventurer who cut it free did not attack it, it will introduce itself, quite politely, as Humbgrol the Erudite, and offer the adventurer a wish for his troubles. If asked about its eating of the other adventurers, Humbgrol will respond that it had been thusly sewn up for decades, and it was quite hungry, indeed, on the verge of starvation.
If asked, Humbgrol informs the adventurer that he is in fact, a demon, placed here long ages ago by a cult. In return for a sacrifice, he is able to answer any one question thoroughly and truthfully, no matter how secret, obscure, or otherwise lost the knowledge may be... though his answer may be in no more than seven words. He can also grant one wish per century.
Finally, Hubgrol will offer the adventurer any remaining magic items or gems that he could not digest from the adventurer's erstwhile companions, in return for the adventurer broadcasting that Humbgrol is again available for service... after all, he needs people to come to him and bring him sacrifices in order to eat!