Medusa

Medusa was once a beautiful lady. She bragged about how beautiful she was to the goddess, Athena. Athena then used her powers to turn Medusa into a hideous monster with snakes for hair. Anyone who looked at her turned to stone. Perseus was sent on a mission to destroy Medusa. He was careful to not look into her eyes (for fear of being turned to stone) so his battle was guided by her reflection in Athena's bronze shield. He cut off her head, placed it in a bag, and gifted it to Athena.
Source: h