Barovia was conquered in the year 347 of Barovia's calendar by Strahd von Zarovich.
Sometime around 351, as detailed in I, Strahd: The Memoirs of a Vampire; a novel published by TSR in 1993 details Strahd's transformation to vampire as a deal gone wrong with a personification of Death itself in an attempt to win Tatyana's love away from his brother.