Medusa is the only mortal one of the three Gorgon sisters, monstrous daughters of the sea deities Phorcys and Ceto, whose gaze turns any onlooker instantly to stone. Later poets, Ovid chief among them, tell that she was once a beautiful maiden whose hair Athena transformed into serpents as punishment after Poseidon violated her within Athena’s own temple — a curse that makes the goddess’s later use of her severed head feel deliberate and pointed.
Perseus is sent to bring back her head as an impossible task set by King Polydectes, and succeeds only with divine help: Athena lends a polished bronze shield to view Medusa’s reflection rather than her deadly gaze, Hermes lends winged sandals, and the nymphs supply a cap of invisibility and a magic pouch. He beheads her while she sleeps, and from her severed neck spring Pegasus and the warrior Chrysaor, offspring she had conceived by Poseidon. Perseus later uses her head as a weapon before giving it to Athena, who mounts it on her aegis, where the staring Gorgon face continues to ward off enemies.
ἣ μὲν ἔην θνητή, αἳ δ᾽ ἀθάνατοι καὶ ἀγήρῳ, αἱ δύο
“She was mortal, but the two were undying and grew not old.” — Hesiod, Theogony