Osybus didn't exist before Van Richten's Guide (except as a kinda joke reference in Death House). His insertion -- and Ulmed's Inquisition's insertion -- into Strahd's history is all new. The Priests of Osybus, which are "monsters" that are provided in VRG, exist as a result of this addition.. Necromancers of deep evil, the priests of Osybus steal the souls of others to fuel the priests' malevolent magic. Using this soul power, each priest can defy death and become an undead creature, potentially cheating the grave over and over.
The Priests of Osybus belong to an old order of magic users that wield necromancy powered by the souls of their victims. They are all marked by the the tattoos of their founder, Osybus, although they hate him because he has cursed them such that their immortality will fail them when their need for it is at its zenith [1].. Back-line priests of Osybus make Soul Blade attacks only if they get blitzed by a charging melee attacker. This pattern changes slightly if a priest of Osybus is struck down in combat and revived by the Dread, Ectoplasmic, Vampiric or Spectral Boon of Undeath.