A cruel demon, who, according to Johan Weyer, filled the post of chief executioner to the monarch of Hades. The conception of him somewhat resembles that of Nemesis. Zoroaster is said to have called him "The Executioner." Others identify him with the destroying angel. Evil genies were formerly called alastors. Plutarch says that Cicero, who bore a grudge against Augustus, conceived the plan of committing suicide on the emperor's hearth, and thus becoming his "alastor."
Weyer, Johannes. Witches, Devils, and Doctors in the Renaissance: Johann Weyer, De Praestigiis. Edited by George Mora. Binghamton, N.Y.: Medieval and Renaissance Texts and Studies, 1991.