Black Guardian:

The Black Guardian is an anthropomorphic personification of the forces of entropy and chaos, the counterpart of the White Guardian, a personification of order. The two Guardians balance out the forces in the universe, although the Black Guardian seems to desire to upset the balance in favour of chaos and evil while the White Guardian prefers to maintain the status quo. The White Guardian gave the Fourth Doctor the task of finding the scattered six segments of the Key to Time, an artifact of immense power that would give the wielder supreme power over all existence. Once the Fourth Doctor had assembled the Key the Black Guardian disguised himself as the White and attempted to trick the Doctor into handing it over. The Doctor saw through the deception after the Guardian appeared indifferent to the fact that the sixth segment,  Princess Astra of Atrios, had ceased her independent existence by becoming part of the Key. The Doctor then dispersed the Key, earning the Black Guardian's eternal enmity. Some time later, in an attempt to destroy the Doctor, he enlisted a young public school student named Vislor Turlough, offering the young man, who was really an alien exiled on Earth, passage home if he killed the Fifth Doctor, and death if he failed. Turlough joined the TARDIS crew, struggling with the dilemma and eventually chose loyalty to the Doctor. Although extremely powerful, the Guardians apparently cannot be seen to act directly, which is why they can only affect things through agents such as Turlough and the Doctor. Turlough's choice and the use of the Enlightenment crystal apparently banished the Black Guardian, who is seen to burst into flames and appear in negative again, before disappearing. The White Guardian warned, though, that he would return, angrier now that the Doctor had thwarted him twice.

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