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. With the White Guardian and four others, he was part of the Six-Fold God known as the Guardians of Time.

Powers
The Black Guardian was the embodiment of chaos in time. Like all Guardians, he was only as powerful as his element in the universe. He was able to change his appearance at will, enter the Doctor's TARDIS, and channel his power of destruction to his follower Wrack. He appreciated creatures with random flaws and perfections, such as vampires.

While he was powerful, theoretically, he could do much more damage with the Key to Time. In addition, he was unable to enter the Doctor's TARDIS when its defences were activated (TV: The Armageddon Factor) and was additionally dispersed by time differential, the Key to Time being broken, and Enlightenment. (TV: Mawdryn Undead, The Armageddon Factor, Enlightenment)

The Black Guardian was the most powerful at the end of the universe where his counterpart could interfere the least, and he was able to follow the Doctor through time and space, seeing his adventures against the Wizard of Avalon, the Animus, Ruath, and the Timewyrm. He could only contact lower primates (like humans and Trions) when their minds were knocked into altered states. (PROSE: The Well-Mannered War)

Being
On one occasion, the Doctor referred to beings sometimes known as "a" Black Guardian, implying that there could be more than one. (TV: The Stones of Blood)

Vampires
The Fifth Doctor theorised that the Black Guardian played some part in the concept of vampires, seeing their potential as creatures of chaos, and somehow being responsible for the law of vampires not becoming "true" vampires until a full moon. (PROSE: Goth Opera)

Prometheus
The Black Guardian, collectively with the other Guardians, removed Prometheus from existence and imprisoned Kronos, then known as Avatar, in the crystal of Kronos and threw it into the Time Vortex. (PROSE: The Quantum Archangel)

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. (TV: 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 posed as the White Guardian and asked for the Key. 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. (TV: The Armageddon Factor)

The revenge
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. (TV: Destiny of the Daleks) After installing the randomiser, he programmed the TARDIS to visit approximately 1,000 planets, which took about a month. He left K9 aboard the TARDIS during this time while he and Romana took up residence in a London townhouse in 1929. (AUDIO: The Auntie Matter)

The Fifth Doctor later piloted the TARDIS to more enjoyable and relaxing destinations, worrying that the Black Guardian would track him down more easily. Incidently, the Guardian had some part in organising Ruath and Yarven's scheme to restore Gallifrey to power in an era after it had ceased to exist, turning humanity into vampires. When they were defeated by the Doctor, he met with the White Guardian and confided that he would need a different method to defeat the Doctor. (PROSE: Goth Opera)

Turlough's assistance
The Black Guardian 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 to 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. (TV: 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. (TV: Terminus) The Doctor and his companions received a warning from the White Guardian about the Black Guardian. They then found themselves in the midst of a contest for Enlightenment, symbolised by a crystal of unknown powers and great value. It was much sought after by the amoral Eternals, who vied for it by racing sailing ships across the Solar System. 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 said the Black Guardian would exist as long as he did, until neither were needed any longer. (TV: Enlightenment)

The second quest for the Key to Time
The Black Guardian rescued the Fifth 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. (AUDIO: The Judgement of Isskar)

After that encounter, the Guardian took them to many places but they could not find the fifth segment. The Doctor realised the Guardian was being methodical in his search; they could find the fifth segment by being heedless of order, their destination 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 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. (AUDIO: The Destroyer of Delights)

Stranded in 9th century Sudan, he was a shadow 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 voice was 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. He had rejuvenated with 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. (AUDIO: The Chaos Pool)

The third quest for the Key to Time
The Black Guardian created an alternate timeline in which the First Doctor had never left Gallifrey, and was now Lord President. However, it also meant that countless aliens had invaded Earth in absence of the Doctor stopping them. The White Guardian sent them on a quest once more for the Key to Time, which could restore the natural timeline. (COMIC: Time & Time Again)