Black Guardian

The Black Guardian is an anthropomorphic personification who embodies both Darkness and Chaos. He opposes the White Guardian.

The Key to Time
The White Guardian set the Doctor and Romana on a quest to find the six segments of the Key to Time, transmuted into a variety of forms and scattered across time and space. He warned them of the Black Guardian who would try to prevent the assembly of the Key. (DW: The Ribos Operation)

The sixth and final segment took the form of a living being, Princess Astra of Atrios. The Black Guardian used the Shadow to try to prevent the Doctor and Romana from re-assembling the Key. After the Doctor defeated the Shadow and re-assembled the Key, including the Sixth segment, formerly Princess Astra, the Black Guardian presented himself as the White Guardian and asked for the Key back. Observing the Black Guardian's casual indifference to the life of Princess Astra, the Doctor saw through this ruse, and scattered the Key through time instead. The Black Guardian swore revenge on the Doctor and Romana. (DW: The Armageddon Factor)

Turlough
The Black Guardian appeared to Turlough, an alien youth exiled to Earth. He promised Turlough that, if he killed the Doctor, that he would return Turlough home. The Black Guardian gave Turlough a small crystal by which he could communicate with him. Turlough ended up joining the Doctor, Tegan and Nyssa in the TARDIS, still at least partly determined to carry out his mission. (DW: Mawdryn Undead)

Turlough began to rebel against the Black Guardian, wishing to back out of the bargain. The Black Guardian still convinced Turlough to sabotage the TARDIS. (DW: Terminus).

The travellers found themselves warned by the White Guardian and found themselves in the midst of a contest for Enlightenment, symbolized by a crystal of unknown powers and great value, as a prize to the winner of a space race undertaken by Eternals. The Black Guardian hoped that, with such a prize, the Eternals might wreck havoc throughout the universe. Instead, the Doctor and Turlough won the race but refused Enlightenment, offering it to Turlough.

The Black Guardian told Turlough that, to get the crystal, he must kill the Doctor. 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 needed any longer. (DW: Enlightenment)