One of the Gorgons, and the only one who was mortal. Her gaze could turn whoever she looked upon to stone. There is a particular myth in which Medusa was originally a beautiful maiden. She desecrated Athena's temple by lying there with Poseidon. Outraged, Athena turned Medusa's hair into living snakes.
Medusa was killed by the hero Perseus with the help of Athena and Hermes. He killed her by cutting of her head and gave it to Athena, who placed it in the center of her Aegis, which she wore over her breastplate.