Building 7 rooms

WERE-CYCAD:       PLANT
SACRED MANTICORE: MONSTER
BLUE MAGGOT: VERMIN
MERCURY GNOLL: HUMANOID
PHASE GIANT: BOSS
TITANIC COBRA: CRITTER

A--D
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G--B--E
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F--C
Room A.
SMELL: Dry and musty, like mummy wrappings and black mold.
SOUND: Creepy singing, like every nursery rhyme in every horror movie ever.
Salt lick. Vein of salt (somehow) in wall of cave, TITANIC COBRAs lapping at it. If you bring salt to other parts of cave the TITANIC COBRAs there will lick it out of your hands and it will be adorable.
Room B.
MERCURY GNOLL herdsmen squabbling over where the sweetest WERE-CYCADS are. Ones in room apparently not sweet enough. Herds of TITANIC COBRAs getting bored, hungry.
Room C.
SOUND: Sound like crunching glass or gnawing teeth. Comes from within walls.
Space-distorting statue in exactly-square corner of corridor. Shaking right hand will increase number of corners until corridor is mile-long side of polygon. Shaking left hand will decrease it again.
Room D.
SIGHT: Fetishes of bone and feathers dangle from the ceiling.
SOUND: Agonizingly slow waterdrip.
Formerly a wizard's study. Now a breeding pair of TITANIC COBRAs has made a nest of the spellbooks, which reveal interesting magical snippets if closely examined. Torn-up pages, gummed back together, may have produced odd new spells.
Room E.
SOUND: Rumble of tectonic plates shifting, impossibly loud and distant.
Single incredibly-intelligent MERCURY GNOLL has found only space in dungeon where it is possible to just sit and think. If left to own devices will rule own kingdom on surface world in ~10 years.
Room F.
SMELL: Meaty reek of underworld flowers, like frying fat and potpourri.
TITANIC COBRA den. Where they keep their babies. Parents not pleased to see you.
Room G.
Looks like empty room. Actually maw of cave leviathan, those weird stalagmites its teeth. Glimmer of treasure deep in gullet is pyrite lure. TITANIC COBRAs pick dead flesh from between teeth.