In Greek mythology the Erinyes (the Romans called them the Furies) were female personifications of vengeance. They were usually said to have been born from the blood of Uranus that fell upon Gaia when Cronus castrated him; i.e., they were chthonic (earth) deities. According to a variant account, they were born from Nyx. Their number is usually left indeterminate, though Virgil, probably working from an Alexandrian source, recognized three; Alecto ("unceasing"), Megaera ("grudging"), and Tisiphone ("avenging murder"). The heads of the Erinyes were wreathed with serpents, their eyes dripped with blood, and their whole appearance was terrific and appalling. Sometimes they had the wings of a bat or the body of a dog.
Tisiphone fell in love with Cithaeron. She caused his death by snakebite, specifically, one of the snakes from her head.
The Erinyes generally stood for the rightness of things within the standard order; for example, Heraclitus declared that if Helios decided to change the course of the Sun through the sky, they would prevent him from doing so. But for the most part they were understood as the persecutors of mortal men and women who broke "natural" laws. In particular, those who broke ties of kinship through patricide, murdering a brother (parricide), or other such familial killings brought special attention from the Erinyes. It was believed in early epochs that human beings might not have the right to punish such crimes, instead leaving the matter to the dead man's Erinyes to exact retribution. The goddess Nike filled a similar role. When not stalking victims on Earth the Furies were thought to dwell in Tartarus where they applied their tortures to the damned souls there.
The Erinyes are particularly known for the persecution of Orestes for the murder of his mother, Clytemnestra. Since Athena had told Orestes to kill the murderer of his father, Agamemnon, and that person turned out to be his mother, Orestes prayed to her. Athena intervened and the Erinyes turned into the Eumenides ("kind-hearted"), as they always did in their beneficial aspects.
As a euphemism, the Erinyes were known as Semnai ("the venerable ones").
The Furies, (their Roman name) or Dirae ("the terrible") typically had the effect of driving their victims insane, hence their Latin name furor.
Virgil VII, 324, 341, 415, 476.