Black Guardian

The Black Guardian is an anthropomorphic personification, embodying both Darkness and Chaos. He used the Shadow to try to stop the Fourth Doctor from claiming Princess Astra of Atrios, the sixth segment of the Key to Time. After the Doctor defeated the Shadow, he assembled the Key and outwitted the Black Guardian, enabling The White Guardian to restore universal balance. (DW: The Armageddon Factor) The Black Guardian swore revenge against the Doctor and later, used tried to use Turlough to try to kill the Fifth Doctor. (DW: Mawdryn Undead, Terminus)

The White Guardian and the Black Guardian offered Enlightenment, symbolized by a crystal of unknown powers and great value, as a prize to the winner of a planet-spanning space race undertaken by Eternals. The Black Guardian hoped that, with such a prize, Eternals might wreck havoc throughout the universe. Instead, the Fifth Doctor won the race but turned down Enlightenment, which was offered instead to Turlough. The Black Guardian reminded Turlough that, to get the crystal, he had to kill the Doctor. But Turlough refused and threw the crystal at the Black Guardian, who vanished in a burst of flames. However, the White Guardian explained that the Black Guardian would exist as long as he did, until neither were need any longer. (DW: Enlightenment)