Third Gorgon

The third Gorgon of three, the first being Medusa, came to Earth in around 1000 BC via a talisman, entering Greek mythology in the process.

The first, Medusa, was slain by a Greek hero; the second was killed sometime in the 1950s by Edgar and Bea Nelson-Stanley, leaving only one.

Sometime in the 1800s, the third Gorgon possessed an abbess, who was host until 21st century. Several generations of nuns protected this Gorgon. The Gorgon's talisman, which Bea Nelson-Stanley had, was hidden in a box that the Gorgon tried and eventually did find in the 21st century. At this time she needed a new host, choosing Sarah Jane Smith. Before the Gorgon could do this, with the talisman it could open the gateway to Gorgos so the Gorgons could invade.

Attempting to engulf Sarah Jane, Maria Jackson used a mirror — with the Gorgon's power reflected back on itself it turned to stone. Maria took the talisman out of its place so the Gorgons could not invade. (TV: Eye of the Gorgon)

Alternate timeline
In Andrea Yates' World, where the Trickster manipulated events so that Sarah Jane Smith died in 1964, the Gorgons never came to Earth. As the Trickster explained to Sarah Jane while she was in limbo, he had turned away all the foes she would have defeated, his intention being for her absence to allow Earth to be destroyed by the meteorite K67. (TV: Whatever Happened to Sarah Jane?)

Behind the scenes
While Eye of the Gorgon names Stheno and Euryale on screen as the other two siblings of Medusa, the story does not make clear which of the second and third Gorgon is which.