The Swamp of Monsters

A great swamp filled with ruined cities, mutant monsters, the mindflayer empire Zin, the Temple of Elemental Evil (where is entombed Zuggtmoy, Queen of Putrescence), Quallmar the Sunken City, and the Gorge, the great seeping crater at the border of Black Abazidun where resides the red dragon Xal-zax Thruul, the Blighted Malice.

Think Fritz Lieber, crossed by canals of toxic mud, interspersed with Labyrinth type environments, Cthulhu star creatures, The Village of Hommlet and other Greyhawk places, and Demon Souls' Valley of Defilement swamp / Tower of Latria.

Monsters

Eelmen
Venus fly trap men
Swamp witches
Hyenamen
Red elves
Mutant dwarves
Toxic people
Traveling merchants
Fungal druids
Panthermen

Frog abominations
Slaad
Mindflayers
Goblins
Shriekers
Shriekers that cast spells
Flying manta rays
T-rex
Yeth hounds (and pits thereof)
Naga
Unicorn head demon
Harpies
Demon with alligators for hands

Black panthers
Crocodiles
Three-headed wolves
Giant frogs
Giant crayfish
Two-headed crows
Cat lizards
Giant snapping turtles
Basilisk lizards
Canal squid
Snakes snakes snakes snakes snakes
Panthersnakes
Giant leeches
Singing swamp slugs

Mud skeletons
Soul vendors on rafts
Tree wights
Rune-mapped royal mummies
Sheliches

Swamp ooze
Black ooze
Red ooze
Green ooze
The amniotic sacs which the abominable children of Xal-zax Thruul birth into the swamp, which in turn incubate a random monster, or suck you up, or both
The Thing-type doppelgangers
Pillars of emerald slime
Trees of ruby slime

Places

Quallmar
Zin
The Vaults of Zin
The Thieves Guild
The Party Barge of Archduke Labriselsis
The Stronghold of Ningauble of the Eyeless Face
The Sin Vats
Swan farms
Hydra factories
Halfling villages build of woodplaster, like wasp hives atop trees
The Palace of Stars, whose powers and disposition shift according to the malicious star in rule
The Fissured Palace of Xal-zax Thruul, The Blighted Malice
Beldr Yeth, the Inverted City of Goblins, and the Labyrinth around it

Hazards

Geneseeking viral spells
Toxic rivers
Cancer sand
The twice-cursed obelisks of Zin
Swamp hexes

Powers in the Swamp of Monsters

Loriana, Vampire Queen of Quallmar
Ningauble of the Eyeless Face
Sheelba of the Seven Eyes
Zuggtmoy, Demon Queen of Putrescence
Pope Verelith IX, Keeper of the Six Elemental Archbishops
The Red Dragon Xal-zax Thruul, the Blighted Malice
Wodney Weck, the Red Star, High Guildmaster of the Thieves' Guild in Quallmar
The Warbands of the Seven Dead Stars
Tujak Z t Od, Mindflayer Priest-King of Zin