Medusa is a Gorgon, who was once a mortal woman, however was punished by the goddess Athena for the crimes of Poseidon. She is known to have a stare which would turn a person to stone upon gazing into her eyes and have hair, made of snakes.
Medusa was once a beautiful virgin woman who worshipped at the Temple of Athena. She was to not ever be touched by any man, as Athena was a virgin goddess. Poseidon however had raped her and instead of taking pity upon Medusa, Athena took Poseidon's side in the matter and punished her. As such, Athena was made a gorgon, who had turned many to stone in her isolation, who came to kill her. It wasn't until the son of Zeus, named Perseus proved victorious against her and killed her, severing her head from her body.