Gorgon

The Gorgons were a species of parasites. (SJA: Eye of the Gorgon)

Biology
The Gorgons had the appearance of ethereal serpents. Each Gorgon was composed of several of these serpentine forms.

They, being parasites, inhabited the bodies of various species. For some reason, they had chosen Humans as their main hosts, despite living 100,000,000 light years away from the Human homeworld of Earth.

The Gorgons had a long lifespan of many thousands of years. Their hosts also had longer lifespans than the normal members of their species, and one host lived for over 200 years.

Gorgons also had the ability to petrify creatures - that is, turn them into a lithous substance similar to stone in a method like that of fossilisation or Petrifold Regression, except quicker (within a matter of hours). (SJA: Eye of the Gorgon)

History
Three Gorgons came to Earth around 1000 BC via a talisman. Due to this, they entered Greek mythology. 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 a sisterhood of nuns, 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 Gorgan whilst trying to possess Sarah Jane as a new host. (SJA: Eye of the Gorgon)