Who turned Medusa into a beast?

Athena Goddess of Intelligent War turned Medusa into a gorgon because she was a nymph that was having an affair with Poseidon God of the Sea. You see Medusa and Poseidon decided to meet inside Athena's temple. They had sex and Athena got angry and since Medusa had her sisters help her gain entrance to the temple Athena transformed them as well, but only Medusa was given immortality so she had to watch her sisters grow old and die.