- "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, who'se ceilings, floors and wallks all have many 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. they rpovide scaring tales ofthe pit, and sometime throw people he can feed upon their fear, and it tests (and nourishes from ) the fear or courage. the idea is to play this as a horror encounter:
- flesh dripping from the ceilings, coming from the floors. at first id' suggest it it mostly appear in small 1-3 hp segments, chattering gibbering)
- the gibbering should be the fears and terrors and secrets of past victims (perhaps someone the PCs were looking for).
- horror/ suspense music is good. try this for example
- the creature tries small attacks to begin with, perhaps receding if it comes close, gibbering "later, when meat is tender".
- it doesn't enable them to sleep, constantly interrupting (or even chattering in their sleep giving nightmares)
- it then start gabbering the PCs secrets, first to each one in their face, then to the rest of PCs, especially if these secrets may have effect (great for roleplay),
- more battles, each time showing more of their selves. might surprise casters by "sliping" right next to them.
- cannot be easily tracked as it hides in walls. they'll have to trick him it into the opn if they want a fair fight.
- at the end either a big finale fight, or just attacking the most fearsome ones, letting the others go.
- the cleaner/ the bargain:
a gibbering mouther has the "slithering trash can" of a dungeon. noticing the PCs it is curious and strike a conversation. it is willing to bargain- info (pass words, secret doors, key vulnerabilites of someone) for... whatever you think gibbering mouther wants ("new fresh bodies? someone fearing and screaming?")
the idea is to make this a very disturbing bargaining. 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? 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. 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 someone related to all disappeared long ago (and was eaten by a mouther), someone who the PCs learn, knows a lot of strange horrific dirty secrets.
the mouther has the person's memories, and has used it's ability to slip to the city (it might move to places through sewers or whatever), and is trying to revisit the horror upon the people (who either committed it or are staying silent about it). the problem is finding and tracking this unconventional killer (that can get almost anywhere, and might leave only strange mark... (bit of flesh? odd bite mark?). the process will probably need to expose the horrors of the potential victims.
- "you're not coming back alive! sane that is."
some far realm or death or assasin cults have taken special precautions to make sure theyir victims don't get back whole. they feed them to a big gibbering mouther. those who were raised/ ressurected and such come back babbling insane or heavily wis damaged.
but these initial killings have been just experiments. now some important person have been killed and fed to the mouther. as the PCs find this out, they fight the cult just as they send the mouther (through some means or creating a great horror somewhere else) to that locations. the PCs can either try and locate the mouther in the new location, or by soem strange means follow it to the plane it slipped to... but what is this place?
- the horde 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 messangers/ fore front/ hounds of some abomination from the far realm, a terror force from mind flayers, or something strange alltogether.
- the split eater comes together:
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 joining up, and increasing in power. some cults might aid them, and they may now seek very specific kind of creatures to devour. this could be an idea for a campaign, at it's end the mouthers are supposed to fomr some great horrific being- i'm thinking of "the eater", which is one great mouth to devour the world, and all it's delicious, sweet terror).