The Fates have the subtle but, awesome power of deciding a mans destiny. The assign a man to good or evil. There most obvious choice is chosing how long a man lives. There are three Fates. Clotho, the spinner, who spins the thread of life. Lachesis, the measurer, who choses the lot in life one will have and measures off how long it is to be. Atropos, she who can not be turn, who at death with her shears cuts the tread of life. The three fates share one eye, taking it out of the socket and giving it to the other to put in.
The Fates are old and predate the gods. It is not entirely clear how far their power extends. It is possible that they determine the fate of the gods as well. In any case, not even the most powerful is willing to triffle with them.