The eldest and immortal Gorgon sister was Stheno.
Stheno was one of the three Gorgons, alongside Euryale and Medusa. In the familiar mythological genealogy, the sisters were daughters of Phorcys, a primordial sea god, and Ceto, a sea monster. Stheno and Euryale were immortal, while Medusa was mortal.
Medusa is the sister who became famous for her petrifying gaze and was killed by Perseus. Guided by Athena and equipped with divine tools, Perseus used the reflection in his shield to avoid looking directly at Medusa. After the beheading, Stheno and Euryale pursued him, but he escaped using the invisibility cap of Hades.
A common mix-up is to treat Medusa as the only Gorgon or to assume all three sisters shared the same fate. In the standard account, Stheno and Euryale survived Medusa’s death. Gorgon imagery also changed over time: early Greek art often showed frightening composite monsters, while later representations increasingly portrayed Medusa as a beautiful woman.