Madness for my Dungeoncrawl

[Segial]

I'm putting together a dungeon crawl where the party has to infiltrate a castle through a network of natural caverns and tunnels beneath the castle. In their preparations the party learned that a former guardsman escaped through the tunnels when the evil forces took the castle, but was driven mad by what he experienced down there. While I have mapped out pretty much most of the dungeon it is still lacking a centerpiece monster/trap/artifact/effect/whatever that would have caused the madness. It would need to be something/someone that would live/exist in a natural cavern enviroment with little intention of leaving or going to the surface, would not be as lethal that a lonely lost guard would not be able to escape it but still challenging for a 10th level party. Any ideas?

[BlueInc]

I think there's an advanced Fihyr (http://monsterfinder.dndrunde.de/details.php?id=541&backlink=%2Fresults.php%3Fid%3D2076853026%26skip%3 D30%26order%3D) in MMII.

Edit: Oh hey, right beside it: Great Fihyr (http://monsterfinder.dndrunde.de/details.php?id=542&backlink=%2Fresults.php%3Fid%3D2076853026%26skip%3 D30%26order%3D)

Edit: AFB and I can't find the full stats online, but basically they're emotion-warping aberrations; I remember using the CR 3 Fihyr against a party and they were awesome at messing with players, terrible in combat. If that trend holds with the CR 15 version, it could be fun.

[NOhara24]

A lich? A buffed Harpoon Spider?

[Telonius]

A group of Carrion Crawlers.
A group of Rust Monsters.
An Elder Black Pudding.

[Kol Korran]

i once used an advanced gibbering mouther that could crawl over walls and ooze itself through really narrow spaces. and at times, instead of gibbering, it would talk, in several voices, like a chorus of mad strange mutated children mouths...

now have the caverns be filled with small holes and tunnels the GM can crawl through, towards nearly every room looking ofr either entertainment or food. the party could hear the gibbering, or mocking/ cryinh/ whimpering/ laughing/ whatever voices coming close and near, maybe catch a glimpse of a fleshy meaty sliver with odd eyes and mouths poking from a hole, or few, at various points in the room.

the party may even converse/ question/ deal/ threat the creature, trying to deal with it's alien mind. and when it does chooses to feed, make it pour from multiple holes to form a big mass of flesh ayes and teeath.

mechanically, i suggest you upgrade it to huge (no matter what the MM says), and maybe upgrade it's attacks, maybe now that it's lager it can attack multiple opponents?

as to the guard- the GM could have been full, deciding in prefer "play" rather than "eat" (it could refer to the guard. "first come one, but he's no fun! so afraid, so silly! you come now! you not silly? you plaaaaayy?") , or the guard just ran fast enough. GMs are slow.

i used a similar encounter once. was brilliant. the player were quite unnerved when it talked in different voices, arguing with itself, or agreeing...