Black Guardian

The Black Guardian, also known as the The Guardian of Darkness and Chaos was an anthropomorphic personification of 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, 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)

The Revenge
In order to escape the Black Guardian, the Doctor fitted the TARDIS with a randomizer to prevent the Black Guardian from knowing where they'd next turn up. (DW: Destiny of the Daleks)

Turlough's assistance
The Black Guardian later appeared to Turlough, a young Trion exiled to Earth. He promised Turlough that, if he killed the Doctor, he would return Turlough to his home, and 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) 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, symbolized by a crystal of unknown powers and great value much sought after by the amoral Eternals, who vied for it in a race against 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, instead 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)

The second quest for the Key to Time
He 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 instructed her. (BFA: The Key 2 Time - The Judgement of Isskar)

After that encounter he then took them to various locations in the universe where the fifth segment to the Key could be found and still couldn't find it, however the Doctor realised that the Guardian was being methodical in his search and that could find the fifth segment by being headless of order and being random, having their destination being controled by chaos, the Guardian sent the doctor and Amy on their way and followed them as soon as he could track them down. 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 be quicker) to create the orium needed, which in fact was the fifth segment of the key to time. This evasion brought the Legate of the Caliph, who was the embodiment of law in he region, to collect the tax, but however brought enough gold to make the orium. When the realised who each other was they couldn't trust each other with the orders the Doctor had given them so ended up both doing the orders. This bickering managed to give enough time for the Doctor and Amy to escape with the segment. (BFA: The Key 2 Time - 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 comunications he thought was Romana's he gave her a telepathic comunication, which he left on the 'answer phone'. The voiced turned out the be President Astra's, a double of Romana, but nonetheless Romana recieved the communication and found The TARDIS in hyperspace. The TARDIS took her to some of its recent destinations, Mars and 9th centuary Sudan, where she found the Black Guardian, who led her to believe that he knew where the Doctor was. When she realised this it was to late, as he had rejuvinated using the power of the sixth segment, Romana herself. Trying to get his final aim, he used the Freedom fighters gathered to stop the White Guardian's teuthoidian hord get the Key and then stopping it form being repaired, causing the universe to fall into chaos. The Grace sent him and his adversary back into their eternal struggle in the howling void. (BFA: The Key 2 Time - The Chaos Pool)