Stheno is the oldest of the three Gorgon sisters; her two younger siblings are Euratye and the infamous Medusa. All three were once extremely attractive young women, with Medusa in particular being the one whose affections men fought for. However, due to her duties as a priestess of Athena, she was bound by a vow of celibacy. This, however, was of little deterrence to Poseidon, who had an affair with her. Athena was furious with her defiled priestess and, while she forgave Poseidon, cursed Medusa and her sisters by turning them into gorgons-snake-haired monsters with a deadly, petrifying gaze.
As the final part of their punishment, Athena had the three cursed sisters exiled to a dismal, isolated island where they would live out the red est of their lives. Humans would travel to the island, but none would ever return. While Stheno and Euratye were immortal and could live forever, Medusa was not, which would lead to her death and the hands of Perseus.