The Carnival
The Carnival is a secret society/location within the Ravenloft setting that can be best described as a kind of floating pseudo-domain in the Demiplane of Dread.
2e
It's a traveling circus that mostly stands out for the fact its members are all incredibly warped, looking like calibans or planetouched or even outright monsters. It actually began as a combination of prison and freakshow in a now-lost domain called L'Morai, where those who were deemed criminals were mutated through fleshcrafting, given amnesia, and tossed into the Carnival L'Morai to serve as victims from the asshole denizens of the domain. The current Carnival began when the freaks rebelled and managed to escape by fleeing into the Mists, something recounted in the novel "Carnival of Fear", where they were found by the mysterious Isolde, who took them under her wing. The Carnival's most notorious feature is "The Twisting", a bizarre magical phenomena that causes anyone who stays overlong in the Carnival's holdings to mutate into a new form that expresses some inner truth about themselves.
In case it's not obvious, this is a full-plot reference to the 1932 film "Freaks," up to and including the Twisting bit.
The Carnival got its own dedicated splatbook/boxed set for Advanced Dungeons & Dragons 2nd edition called, literally, "The Carnival". Fans also f leshed out the Carnival's ranks in various Books of S and Quoth the Raven netbooks.