Ravenloft began as a 1983 adventure module by Tracy and Laura Hickman and opened out into a full setting in 1990. It is set in the Demiplane of Dread, a scatter of small domains divided by mist. Each domain has a Darklord, and Count Strahd von Zarovich is the best known of them.
One rule separates it from other fantasy settings. A Darklord receives the domain as a punishment: they rule it and they cannot leave it. The greater the power, the tighter the confinement. The mists will let people in without deciding whether to let them out.
This setting is where tabletop role-playing worked out how to run horror. Winning stops being the goal; reading the rules of the place you are shut inside becomes the goal instead.