Mara

The Mara was an entity created from the evil from the minds of the Manussans it was given independant life via the Great Crystal. It led the Sumaran Empire, some time following this it was drawn to the planet Deva Loka where it possessed several of the Deva Lokians. When Tegan Jovanka fell asleep on Deva Loka the Mara was able to possess her through her unconscious. Using Tegan's body the Mara sought to create chaos by pushing the Kinda to attack the Earth force on the planet. The Fifth Doctor used a circle of mirrors to make the Mara face itself and therefore make it leave Tegan and be banished once more. (DW: Kinda)

The Mara however was not banished from Tegan and forced her to return to the planet Manussa, the planet where it had been created 500 years ago, it planet to recorporalise itself using the Great Crystal, however the Doctor once more defeated it, this time using then smashing the Great Crystal. (DW: Snakedance)