Mara

The Mara was an entity created from the evil in the minds of the Manussans and given independant life via the Great Crystal. It led the Sumaran Empire. Some time after this, it was drawn to the planet Deva Loka, where it possessed several of the Deva Lokans. When Tegan Jovanka fell asleep on Deva Loka, the Mara possessed 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 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)

However, the Mara was not banished from Tegan and forced her to return to Manussa. It planned to recorporalise itself using the Great Crystal, but the Doctor once more defeated it, this time destroying it permanently by smashing the Great Crystal. (DW: Snakedance) The Tenth Doctor mentions the mara in Time Crash when he is trying to work out where the Fifth Doctor is taken from.