The Mara

The Mara was brought into being when the Manussans created a crystal that could transform energy into matter. Formed from the dark side of Manussan psyche, the Mara took control of bodies of its victims, and from there could enter the physical universe in the form of a giant snake. It was eventually banished into a dream world from which it could not escape, as it had no form. Tegan, dreaming under the mysterious Kinda windchimes, has her dreams are invaded by the Mara, which persuades her to allow it to use her body. She awakens with a snake tattoo on her arm and the Mara is in control. The Mara then migrates to a Kinda native, nearly causing the destruction of the idyllic world of Deva Loka before the Doctor banishes it back to "the dark places of the inside." Later the Mara again takes control of Tegan, intending to use the Manussans' celebration of its banishment as a means to recreate itself. Luckily the Doctor is again on hand to destroy the creature forever. 

The Mara appeared in Kinda and Snakedance