Black Guardian

The Black Guardian, also known as the Guardian of Darkness and Chaos, was an anthropomorphic personification of the forces opposed to the powers of light, as embodied by the White Guardian. Together with the White Guardian and four others, he was part of the Six-Fold God known as the Guardians of Time.

The first quest for the Key to Time
The White Guardian sent the Fourth 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, 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)

The Revenge
In order to evade the Black Guardian's revenge, the Doctor fitted the TARDIS with a randomiser to prevent the Black Guardian from knowing where they would materialise next. (DW: Destiny of the Daleks) He would later remove it and leave it on Argolis. (DW: The Leisure Hive)

Turlough's assistance
The Black Guardian later appeared to Turlough, a young Trion exiled to Earth. He promised Turlough that, if he killed the Fifth Doctor, he would return Turlough to his home. He gave Turlough a small crystal with 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) As he grew fonder of the Doctor, Turlough began to rebel against the Black Guardian, wishing to back out of the bargain. The Black Guardian still convinced Turlough to sabotage the Doctor's TARDIS. (DW: Terminus). The travellers found themselves warned about the Black Guardian by the White Guardian and found themselves in the midst of a contest for Enlightenment, symbolised by a crystal of unknown powers and great value, much sought after by the amoral Eternals, who vied for it in a race across the Solar System in sailing ships. The Black Guardian hoped that, with such a prize, the Eternals might wreak havoc throughout the universe. Instead, the Doctor and Turlough won the race. The Doctor refused Enlightenment, offering it instead 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)

The second quest for the Key to Time
Turlough rescued The Doctor and Amy from an Ice Warrior ship which was plunging into a sun. According to Amy, he was not meant to get involved in their search as the Grace told her. (BFA: The Judgement of Isskar)

After that encounter Turlough took them to places but they could not find the fifth segment. The Doctor realised that the Guardian was being methodical in his search and that they could find the fifth segment by being heedless of order and being random, having their destination being controlled by chaos. The Guardian sent the Doctor and Amy on their way and followed them. In 9th century Sudan, he inserted himself into the time stream and became part of the events. As Lord Cassim, he evaded taxes in order to fix the Djinni's warp manifold, which needed gold, (although platinum or plutonium would have been quicker) to create the orium needed. This was the fifth segment of the key to time. This evasion brought the Legate of the Caliph, the embodiment of law in the region, to collect the tax. In the confusing welter of events, the Doctor and Amy were able to escape with the Key. (BFA: The Destroyer of Delights)

Stranded in 9th century Sudan, he was a wreck of his former self, unable to become what he once was. When the Doctor became trapped on the Teuthoidian ship, he heard a voice on intra-ship communications which he thought was Romana's, and sent a telepathic communication to her 'answer phone'. The voiced turned out the be President Astra's, a double of Romana. Romana received the Doctor's answer and found The TARDIS in hyperspace. The TARDIS took her to some of its recent destinations, Mars and 9th century Sudan, where she found the Black Guardian, who led her to believe that he knew where the Doctor was. By the time she realised this wasn't true it was too late, as he had rejuvenated using the power of the sixth segment: Romana herself. Trying to achieve his final goal, the Black Guardian used the gathered freedom fighters to stop the White Guardian's Teuthoidian horde from getting the Key, locking the two Guardians in opposing balance once again. At that moment the Grace reappeared and sent the two of them back to howling void, to continue their eternal struggle. (BFA: The Chaos Pool)