Medusa ‘s story began when she slept with Poseidon in a "forbidden" place, which was either a temple of Athena or a flower meadow-a place where seduction frequently took place in mythology. For this sexual transgression, Athena turned Medusa from a beautiful-haired, young woman into a serpent-haired monster with boar's tusks, a beard, a protruding tongue, wings, and a gaze so horrible that it could turn onlookers to stone. But Athena's wrath did not end with this metamorphosis. When Perseus accepted the quest for the head of the Gorgon, Athena helped him decapitate the sleeping monster.Medusa, who was mortal, had two immortal sisters, who shared her monstrous appearance.
These were Steno ("mighty") and Euryale ("far-leaping"). Along with Medusa ("cunning"), these daughters of the primordial sea gods, Phorcys and Ceto, were depicted as dwelling at the extreme end of the world, beside the streams of Ocean.by Susan Deacy, 30-SecondMythology