Hungry Mush-Room

A large room containing several alcoves, each of which was fully coated in a carnivorous fungus of some sort. The mush-room put weird, psychic, fungal mullets on each of us, which created a sort of shared delusion in which Kreeree and Tork were the monarchs of the Mushroom Kingdom, and the rest of the party were hailed as "travelers" who--if they survived all 5 trials--would lead the kingdom back to the surface.

It's hard to say what would have happened if they'd failed any of the first four trials, but the fifth and final trial was "The Chamber of the Seals." The chamber was, itself, the hungry mush-room, and each of the "seals" covered one of the alcoves. The Tiefling broke all 15 seals simultaneously, and they all appeared to reveal brightly lit paths back to the surface world. Which was, of course, all a trick to make us walk ourselves directly into the carnivorous fungus.