alastor (n.)

"In early Greek mythology, the spirit of revenge, that prompts the members of a family to commit fresh crimes to obtain satisfaction" [Encyclopaedia Britannica, 1941]. The name also was used of the evil genius which drives a man to sin and of a man so driven. A Greek word of uncertain origin. The traditional guess is that it is literally "the unforgetting," from a- "not" (see a- (3)) + root of lathein "to forget," aorist of lanthanein "to lie hidden, escape notice," from PIE root *ladh- "to be hidden" (see latent). Or else it might be connected with alaomai "to wander, roam," figuratively "to be distraught." As a proper name, in Greek tradition a son of Neleus and Chloris; brother of Nestor, he was slain by Herakles.