Snakedance — Terrance Dicks

A novelization of a terrifying adventure for the Fifth Doctor, Tegan and Nyssa. The TARDIS arrives on the planet Manussa, about to commemorate its ancient banishment of the evil entity known as the Mara. Unbeknownst to the Manussans, the Doctor's companion, Tegan, has become an unsuspecting medium for the Mara. Now she has helped it to return home. The Doctor realizes that this could mark the spectacular revival of the Mara's reign of terror - but it seems that no-one will heed his warning.