Salvager and proprietor of Singer Salvage Yard, Bobby Singer was first introduced to the supernatural world when his wife, Karen, became possessed by a demon. As detailed in the third-season episode "Dream a Little Dream of Me", he was uncertain of how to save her, and instead ended up being forced to kill her.
Ten years after her demon deal, as promised, Azazel entered her home and poisoned Sam with demon blood. Although Mary tried to stop the demon, he burned her alive. John, who is oblivious to the supernatural world, witnessed her death, triggering a lifelong obsession with hunting.