"Pit of the eater":
Either the party has been captured by primitives, or they are required to prove their courage in a test. They are lowered/ thrown down to a dark pit, whose ceilings, floors and walls all have many small holes.
They are required to survive 5 days in the pit (or as you wish). The pit has many rats and possibly some other animals (Which the mouther sometime consumes in order to regenerate) and walls upon which are scribbled frightened rantings of past victims.
The mouther here has made a deal with the tribe above- they provide scaring tales of the pit, and sometime throw people who'se fear he can feed upon, and it tests (and nourishes from ) the young warriors who seek to become full warriors of the tribe. The idea is to play this as a prolonged, yet time set horror encounter. Can they survive their time with the mouther? Some ideas:
- At first, the party may see the rantings of past victims/ warriors trapped in the pit. These should look insane, mad, fearful.
- At times, have flesh dripping from the ceilings, coming from the floors. These may be time segments (Up to 1-3 hit points), of the mouther looking upon the new arrivals. The tiny tendrils of flesh, with an eye or two, mouth or two, should start building up the anxiety.
- As the gibbering starts, feel free to some creepy disconcerting music such as this!
- At times, the gibbering will include fears and secrets, either random, or of NPCs the party may have been looking for, and of the PCs themselves!
- The mouther seeks to invoke fear. At start he attacks the party by small segments, only a little, and then melts and receds into the many crevices, possibly gibbering "later, when meat is tender".
- Harass them. It won't enable them to sleep, constantly interrupting (Or even chattering in their sleep giving nightmares)
- Once it sensed enough of the PCs fears and secrets, it starts jabbering about them, in front of the other PCs, in front of them, exposing their hidden thoughts, desires, shame and more...
- In following attacks it uses more of it's powers, and uses greater segments of itself. Remember it can swim, climb, melt, and even teleport short distances. Put in the pit some places of "supposed safety", only to have the Mouther access them still (Building up hope, then crushing it!
- Even if the party damages it, it can recede to a safe place where the tribe may sacrifice to eat some meat to devour and regenerate.
- It is VERY hard to track down, since it's very hard to follow it, through it's tunnels.
- Near the end of their alloted time, the mouther attacks more in earnest, seeking mostly to engulf a single PC and devour it. It may use parts of it to "run interference". If it DOES devour a PC, have part of it's jabbering reflect the trapped tormented soul, when next they meet...
The cleaner/ t|The bargain:
A gibbering mouther acts as a "slithering garbage disposal" of a dungeon. Noticing the PCs it is curious and strikes a conversation. It is willing to bargain- info (pass words, secret doors, key vulnerabilities of someone) for... whatever you think gibbering mouther wants (Perhaps new flesh or minds to taste? Invoking some terror in a specific individual? Something else?)
The idea is to make this a very disturbing bargaining, with an alien mind/ alien minds, which are... just really really, REALLY not ok. I suggest switching up your voice, your manner of speaking, and whom you're talking to every 5-6 10 seconds or so, or at times mimic a chorus of different voices. ("Yes we shall! No, don't! Why the tasty meat speak? It's been so long, so long! Listen, do you hear? I told her, but she wouldn't listen!") and also be very weird in what you ask, make the party doubtful if they want to make the bargain. I ran this sort of encounter with my party, and they were squirming by the time it ended.
An unexpected murderer:
The PCs are requested to investigate some murders in the city/ big manor/ whatever. In the course of investigations they find that the connecting link between all the people is someone who was related to all of the victims disappeared long ago has gone missing/ been away.
The person was eaten by a gibbering mouther, and now with parts of the person's memories, it used it's ability to slip to the city (it might move to places through sewers or whatever), and is trying to inflict horror and a weird sort of punishment, upon people whom this person was connected to. The problem is finding and tracking this unconventional killer (That can get almost anywhere, and might leave only strange marks... (Bit of flesh? Odd bite mark?).
An interesting twist may be that the people killed were pretty horrendous themselves, and as the party investigates/ faces the mouther at times, or potential future victims, the party comes to realize this, and that the consumed memories of the eaten person seeks to "punish" them. Who's more horrific? The victims? Or the victimized killer? What punishment is too much?
"You're not coming back alive! Well, not coming back sane that is."
Some far realm or death or assasin cults have taken special precautions to make sure their victims don't get back, or at least not as they were. They feed them to a big gibbering mouther, since those who ARE raised/ resurrected and such come back babbling insane or heavily wis damaged.
But what if these initial killings have been just experiments? Now some high profile/ very important person has been killed and fed to the mouther. As the PCs find this out, they fight the cult just as they send the mouther to another location, to ensure the consumed soul isn't available. (They might go to great lengths such as creating a significant horror somewhere distant, or any other means) Can the PCs reach the same location, and locate the mouther? Or will they even find a way to slip where it does, and track it through that realm?
The coming of terror:
Gibbering mouthers are the markers of things gone terribly wrong. suddenly, in a local/ country/ more they start appearing, more and more and more...
You can play them off as an early warning sign, trying to hunt clues from gibbering mouthers, figure out who they killed, or you can take it further- the gibbering mouthers are part of a greater calamity- they are the messengers/ fore front/ hounds of some abomination from the far realm, a terror force from mind flayers, or something strange altogether.
The eater of the whole:
As suggested in the mysteries section- what if the mouthers are but parts of a greater being? this could be in continuation with the former idea, or on it's own.
The mouthers are doing something bizarre- they are joining up, and increasing in power. Some cults might aid them, and they may now seek very specific kinds of creatures to devour, or any other peculiar change in behavior. This could be an idea for a campaign, at it's end the mouthers are supposed to form some great horrific being- Possibly an elder evil or such, "The Eater of the Whole", Which in essence seeks to devour all creation, or "at least" create a never ending horror to draw it, and to feed it's massive appetite...