Medusa

Medusa ("guardian, protectress") was a monster, a Gorgon, generally described as having the face of a hideous human female with living venomous snakes in place of hair. Gazing directly upon her would turn onlookers to stone. Most sources describe her as the daughter of Phorcys and Ceto, though the author Hyginus interposes a generation and gives Medusa another chthonic pair as parents. Medusa was beheaded by the hero Perseus, who thereafter used her head as a weapon until he gave it to the goddess Athena to place on her shield. In classical antiquity the image of the head of Medusa appeared in the evil-averting device known as the Gorgoneion. 

Medusa was the only one of the three Gorgons who was not immortal. King Polydectes sent Perseus to kill Medusa in hopes of getting him out of the way, while he pursued Perseus's mother, Danae. Some of these myths relate that Perseus was armed with a scythe from Hermes and a mirror (or a shield) from Athena. Perseus could safely cut off Medusa's head without turning to stone by looking only at her reflection in the shield. From the blood that spurted from her neck and falling into the sea, sprang Pegasus and Chrysaor, her sons by Poseidon. 

Perseus is said by some to have given the head, which retained the power of turning into stone all who looked upon it, to Athena. She then placed it on the mirrored shield called Aegis and she gave it to Zeus. Another source says that Perseus buried the head in the marketplace of Argos. According to other accounts, either he or Athena used the head to turn Atlas into stone, transforming him into the Atlas Mountains that held up both heaven and earth. He also used the Gorgon against a competing suitor. Ultimately, he used her against King Polydectes. When Perseus returned to the court of the king, Polydectes asked if he had the head of Medusa. Perseus replied "here it is" and held it aloft, turning the whole court to stone.