Medusa

There are two Medusas in the sources, and the difference between them is the whole argument. In the older, Hesiod's Medusa is simply one of three Gorgon sisters, daughters of the sea-gods Phorcys and Ceto, born monstrous — the only mortal one among them, which is the detail that dooms her.

Ovid tells it differently and far more famously. In his version Medusa was a beautiful mortal woman, a priestess in Athena's temple, and Poseidon assaulted her there. Athena, enraged at the desecration of her sanctuary, punished the woman rather than the god: she turned Medusa's hair to serpents and made her face so terrible that anyone who met her eyes turned to stone.

Perseus was sent to fetch her head by a king who wanted him dead and assumed the errand would do it. He was equipped by gods for the job — winged sandals, a cap of invisibility, an unbreakable blade — and approached her using a polished shield as a mirror, so he never had to look at her directly. From her severed neck sprang the winged horse Pegasus and the warrior Chrysaor, children of Poseidon she had been carrying.

The head kept working after death. Perseus used it as a weapon, then gave it to Athena, who fixed it to her aegis. The image outlived the story: a Gorgon's face was carved over doorways across the Greek world as a ward, a monster set to guard against everything else.