Gorgon

The Gorgons were a species of intelligent parasitic beings which possessed their hosts.

Biology
Gorgons had the appearance of blue, ethereal serpents. Each Gorgon was composed of several of these serpentine forms. Though they were incapable of speech (making only hissing noises), they had some telepathic powers, specifically mind control.

Gorgons had the ability to petrify creatures within a matter of hours. In order to do so, the serpents needed to leave the host and make contact with the target. They could be reflected by a mirror, causing them to return to the host and turning it to stone.

Gorgons were a species of parasites, inhabiting the bodies of creatures like Humans. While in the host, they greatly increased its lifespan, letting one Human host live for over 200 years. This was much less than the Gorgon's natural lifespan of thousands of years, which meant they had to frequently switch hosts. (SJA: Eye of the Gorgon)

History
"Serve the Gorgon, Serve the Gorgon, Serve the Gorgon."

- Eye of the Gorgon

Three Gorgons came to Earth around 1000 BC via a talisman, entering into Greek mythology along the way. One Gorgon was slain by a Greek hero, another by a professor and his wife in 1959, and one survived.

This Gorgon eventually inhabited the body of an abbess and led the sisterhood of nuns at St. Agnes Abbey, manipulating their minds to serve it for over 200 years. The Gorgon led the nuns on a mission to retrieve the talisman so as to call the Gorgons from their homeworld so as to invade Earth and take all of Humanity for their hosts. The Gorgon was eventually defeated, however, by Sarah Jane Smith, Luke Smith, Maria Jackson and Clyde Langer using a mirror to defeat the Gorgon whilst trying to possess Sarah Jane as a new host. (SJA: Eye of the Gorgon)

Other information

 * The Doctor had previously encountered a different kind of Gorgon. (TVC: The Gaze of the Gorgon)
 * Hades had some kind of gorgons in his employ. (PDA: Deadly Reunion)
 * Eugene Jones believed that he had some "pre-Gorgon" Pilurian currency in his alien artifact collection. (TW: Random Shoes)

In City of Death the house where Count Scarlioni lived had an emblem of the gorgon on the door.