Gorgon (Doctor Who)

The Gorgon was a lone member of a parasitic alien race from the Sarah Jane Adventures and was the primary antagonist of the "Eye of the Gorgon" storyarc - inhabiting the host body of the Abbess and using the nuns of St. Agnes Abbey to try and fulfil its goal of obtaining a talisman that would allow it to open a portal to Gorgos, the home-world of the Gorgons and allow the aliens to invade Earth and presumably wipe out the human race (after enslaving them and using their bodies as vessels).

Abilities
The Gorgon, like all beings in its species, is a collective mass of ethereal serpents that must inhabit a humanoid body to survive - once inhabiting a body they enhance it considerably, allowing some hosts to live well into the hundreds but not preventing the inevitable decay of said host, thus they must frequently switch bodies as Gorgons live for thousands of years.

Gorgons have mind-control abilities and can petrify victims into stone but must leave their host temporarily to do so, tending to do so via the eyes and mouth - this is also the Gorgons major weak point as reflective surfaces such as mirrors can deflect the Gorgons energy back at their host and effectively kill them via petrification, without said host the Gorgons also perish.