GORGON

Planet of Origin: Gorgos ((100,000,000 light years from Earth)

The Gorgons were a species of intelligent parasitic beings which possessed their hosts. Non corporeal, individuals look like a group of blue ethereal serpentine forms hovering in the air. They had some psychic powers, specifically mind control and the ability to petrify creatures within a matter of hours. In order to do so, it needs to leave its host and make contact with the target. They can be reflected by a mirror, causing them to return to the host and turning it to stone. 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.

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 up til 2010. 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 home world to invade Earth and take all of humanity for their hosts.

Source: Doctor Who Universe (SJA: Eye of the Gorgon)