Alastor ("avenger") in Greek mythology:, was the personification of familial feuds. He was also associated with sins that pass down from parent to child. As a genius, or spirit of the household in Roman mythology, he incited people to murder and other sins. He was originally a mortal, son of Neleus, King of Pylos. He was later downgraded to a minor demon after he and his brothers were killed by Heracles.
In Christian demonology, Alastor became chief executioner to the monarch of Hell. He was likened to Nemesis and the destroying angel and was known as The Executioner. He was exceptionally cruel.
Evil spirits were also known as Alastors.