The New International Encyclopædia/Alastor
ALAS′TOR (Gk. Ἀλάστωρ). (1) An avenging, haunting spirit. Among the Greeks the name was sometimes applied to Zeus as an avenging deity; also to the Furies. In the Middle Ages it was given to one of Satan's chief ministers, a demon supposed to execute his purposes.
(2) A poem by Shelley published in 1816, entitled in full, Alastor, or the Spirit of Solitude.