The physical personification of the forces of chaos and destruction in the universe, the Black Guardian is ranged and balanced against the White Guardian’s force of order. When the White Guardian tasked the Fourth Doctor with reassembling the Key to Time, the Black Guardian’s agents were close at hand. After Princess Astra sacrificed her life to become the sixth and final segment, the Black Guardian posed as White to take the Key. His casual disregard for human life enabled the Doctor to see through the ruse and destroy the Key. Later, the Black Guardian tried to use Turlough to destroy the Fifth Doctor, luring him with a promise to return him home. When faced with a choice, Turlough hurled the Enlightenment crystal at the Black Guardian, consuming him in flames. However, as the White Guardian warned, both Guardians would continue to exist as long as they were needed.