Secrets of Barovia
Gustav and Elizabeth Durst, the parents of Rose and Thorn, had attempted to summon a demon to grant everlasting life, and had pleaded with Strahd von Zarovich to intervene. Strahd had refused via a letter. Due to their cannibalistic natures, Gustav and Elizabeth became Ghasts while many of their fellow cultists became ghouls. Baby Walter’s stillborn corpse was transformed or added or somehow conjoined with a monstrous undead creature that consisted of most of the bones of those sacrificed by the Dursts.