They are the goddesses that avenge the violations of the natural order, including such is inhumanity,(ie. murdering your own father).They rose from the blood of the mutilated Uranus, when it hit the mother Earth (Gaea). They were called the Eumenides by the ancients, which meant "the kindly ones" because if they refered to them as the Furies or the Erinyes "the angry ones", the furies would make them go insane. There are three Furies:
Alecto
The unresting
Megaera
The jealous
Tisiphone
The avenger
They are often seen as being hideous old women with bloodshot eyes and snakes for hair.They punish people by arousing frenzies of remorse. The very sight of them drove a guilty man insane.
