Ravenloft is an alternate time-space existence known as a pocket dimension or demiplane, called the Demiplane of Dread, which consists of a collection of land pieces called “domains”, brought together by a mysterious force known only as the Dark Powers. Each domain is tailored to and mystically ruled by a being called a Darklord who is forever trapped and surrounded by magical mists surrounding the domain. Strahd von Zarovich, a vampire in the original AD&D Ravenloft I6 module 1983, became the first Darklord, both ruler and prisoner of his own personal domain of Barovia. How Count von Zarovich became the darklord of Barovia was detailed in the novel I, Strahd: The Memoirs of a Vampire. As originally established in the Ravenloft: Realm of Terror boxed set known as “the Black Box” released in 1990, the Ravenloft campaign setting was located in the Ethereal Plane. As a physical manifestation of that plane, lands, monsters and even people were created out of the mysterious mists, and the realm acted as a prison where one could enter or be transported, but means of escape were few. Other Ravenloft Domains and Darklords were eventually added in various AD&D 2nd edition (and then later in 3rd edition) products establishing a core continent attached around Barovia which could be traveled to by others if their respective lords allowed entering or leaving their borders; while some Domains remained isolated in the mists and were referred to as Islands.
