Short answers, pulled from the story.
Tracy and Laura Hickman wrote adventures that were eventually published as the Dungeon & Dragons modules Pharaoh and Ravenloft. They play-tested their frightening version of a vampire creature with a group of players every Halloween for five years on their own game system under the title Vampyr.
The first appearance of the setting was in Ravenloft, a stand-alone Advanced Dungeons & Dragons adventure module published in 1984. In 1990, Ravenloft launched as a full-fledged campaign setting for AD&D 2nd Edition with the Realm of Terror boxed set popularly known as the Black Box.
The Dark Powers are a malevolent force who control the Demiplane of Dread with their exact nature and number deliberately kept vague allowing plot development according to Gothic tradition. Heroes frequently find themselves outclassed and outnumbered by unknowable evil forces beyond their control while the punishment cycle pushes Darklords to further evil raising questions about whether the powers deliver just punishment or propagate evil.
Van Richten's Guide to Ravenloft published the 18th of May 2021 includes overview of 39 Domains with wholesale revisions applied to many. Barovia remains the first Domain foreboding home of Strahd von Zarovich inspired by Bram Stoker's Dracula.
They were originally stereotyped as uncivilized heavy drinkers before removal in Curse of Strahd Revamped module released the 20th of October 2020. Their portrayal was further retconned in Van Richten's Guide to Ravenloft published the 18th of May 2021 making them no longer considered superstitious instead focused on traditional practices grounded in D&D magic forms.