The three Furies were horrible. They usually were shaped as crones,with snakes for hair, dog’s heads, coal-black bodies, bat’s wings, and bloodshot eyes. But because theywere older than Zeus, their powers were a mystery. It was also said they could assume beautiful formswhen it suited them. — The Immortal by Christopher Pike

The Furies, also called the Erinyes and for all intents and purposes  the Poinai (Poenae) (Retaliations), Arai (Arae) (Curses), Praxidikai (Praxidicae) (Exacters of Justice) and Maniai (Maniae)(Madnesses) (thank you theoi.com), were the three goddesses of vengeance. Victims of crimes often called down the curse of the Furies on criminals who hurt them.

They were terrifying looking. Depictions feature them as hideous women with wings and covered in poisonous serpents. The carried whips and dressed as either mourners or hunters. The best description I’ve ever read of them came from The Immortal by Christopher Pike.

The furies were born at the same time as Aphrodite. When Cronus castrated Uranus, Aphrodite was born from the “sea foam” that dripped into the sea and the furies were born from the blood. In other versions of the myths they’re daughters of Nyx and Erebus or Hades and Persephone. In other versions they aren’t goddesses at all, but a curse to be cast down on villains.

They were named in some version of the myth. Alecto, Megaera, and Tisiphone (who was also sometimes Demeter in horse form. Long story).

While they worked part time torturing souls in the Underworld, The Furies took delight in their job making living criminals miserable. Entire nations were brought to their knees for harboring their targets. Their wrath could only be stopped with a specific ritual and the completion of an assigned task. Failing that, and the Furies might haunt your family for generations.

At least until Orestes came along.