Guardian of Time

The Guardians of Time, usually simply referred to as Guardians and also known as the Council of Guardians or the Six-Fold God, were masters of reality, embodying aspects of the universe. They were the upper echelons of the Great Old Ones, a pantheon within a pantheon. Despite being at the pinnacle of reality, they would defer to "Greater Old Ones" in some matters. (PROSE: Divided Loyalties, The Quantum Archangel)

Although numbering six in total, the only Guardians ever visibly active were the Celestial Toymaker, and the Black and White Guardians who claimed to each be responsible for 50% of the universe. (AUDIO: The Destroyer of Delights)

Guardians

 * White Guardian of Light and Order/Structure
 * Black Guardian of Darkness and Chaos/Entropy
 * Crystal Guardian of Dream and Fantasy
 * Red Guardian of Justice and Truth
 * Azure Guardian of Equilibrium and Balance and its twin
 * Gold Guardian of Life and Death

A fictional Beige Guardian once appeared to force the first eight incarnations of the Doctor to defeat all their past enemies at once, but it was actually just a video game Izzy Sinclair was playing on the Time-Space Visualiser aboard the Doctor's TARDIS. (COMIC: Happy Deathday)

Nature
Physically, the Council of Guardians existed in Calabi-Yau Space. When together, they were the Six-Fold-God of the Six-Fold-Realm and were able to reshape space/time anyway they wanted. (PROSE: The Quantum Archangel) The Guardians were considered to be immortal, but were capable of being destroyed. When Turlough believed the Black Guardian destroyed, the White Guardian explained that they would always exist until the universe no longer needed them.

Though they could assume a form for a brief period as an "interface" with the universe (AUDIO: The Destroyer of Delights) the Guardians and the other Old Ones' natural form was a collective consciousness, possessing neither form nor substance and existing between dimensions. (PROSE: Divided Loyalties)

Powers
The Seventh Doctor stated that they were capable of altering time on a grand scale and that the power they represented could not exist on something as small as a planet. (PROSE: Blood Heat, COMIC: Time & Time Again) While the Eighth Doctor claimed they did not have the power to stop the universe from expanding, (PROSE: Dominion) the Seventh Doctor said they were one of the powers of the Omniverse that could do anything, hinting they could pull the arms off spiral galaxies. (PROSE: No Future)

More specific powers of the Guardians included:


 * Transmogrification, changing their shape at will and usually assuming a form familiar to those who saw them (TV: The Armageddon Factor, Mawdryn Undead)
 * Hypnosis (TV: Mawdryn Undead)
 * Telekinesis (TV: Terminus)
 * Omniscient-like knowledge, including information on planets, transmat capsules, TARDISes, and Terminus (TV: Terminus)
 * Penetrating a TARDIS defence shield (TV: The Ribos Operation, PROSE: The Shadow of Weng-Chiang)
 * Controlling a TARDIS and absorbing its energy (TV: Enlightenment)
 * Channeling power through an individual (TV: Enlightenment)
 * Following an individual through time and space (PROSE: The Well-Mannered War)
 * Moving individuals through time and space (PROSE: The Well-Mannered War)
 * Creating force fields (AUDIO: The Destroyer of Delights)
 * Great physical strength (AUDIO: The Chaos Pool)
 * Flight (AUDIO: The Destroyer of Delights)
 * Controlling large groups (AUDIO: The Chaos Pool)
 * Freezing time (AUDIO: The Destroyer of Delights, The Chaos Pool)

In terms of weaknesses, the Black Guardian was affected when the TARDIS's defensive systems were fully activated, reshaping into his negative form for unexplained reasons. He could also be dispersed by releases of energy such as the breaking of the Key to Time, (TV: The Armageddon Factor) the shorting-out of time differential, (TV: Mawdryn Undead) or contact with Enlightenment. (Enlightenment) Both he and the White Guardian were also sent back to their struggle in the "echoing void" by the Grace. (AUDIO: The Chaos Pool)

Technology
Though the Guardians seemed to be above traditional technology, the Fourth Doctor mentioned "Guardian technology" in reference to the first Key to Time, (TV: The Armageddon Factor) which existed at every point in time. (PROSE: The Well-Mannered War) Apart from maintaining the equilibrium of time itself, the Key respresented the power of the Guardians and was also used to erase the memory of the Millennium War and to trap Kronos in his crystal. (PROSE: The Quantum Archangel

Additionally, the core to the Key to Time was used to track the segments, which were tied to the Guardians who could draw energy from them. Should the segments start to decay, the Guardians could feel the effects of entropy. (AUDIO: The Destroyer of Delights, The Chaos Pool)

The Black Guardian made use of a crystal that represented a contract with his agents. (TV: Mawdryn Undead, PROSE: The Well-Mannered War)

Interactions with other beings
The Black and White Guardians drew strength from their respective elements, becoming stronger or weaker depending on the state of their elements in the universe. The Black Guardian was most powerful in the far future, at the end of the universe where his counterpart could not interfere. (TV: Enlightenment, AUDIO: The Destroyer of Delights, PROSE: The Well-Mannered War) He created wars to justify his existence, while the Guardian of Justice created conflict. The Toymaker explained that he was in this universe because everyone in the multiverse had dreams and that he shaped them least they become stale. (PROSE: Divided Loyalties)

The Eternals knew of the Guardians and respected them greatly, calling them "Enlighteners" as they set up the games that kept the Eternals amused. (TV: Enlightenment)

Like the other Old Ones, the Guardians originated in another universe. Rassilon himself considered the Old Ones "sub-Guardians" and asked questions of the "Guardians of the Universe." They refused to answer him, saying the Time Lords were superior to the rest of the universe, but there were creatures far superior to them. This taught him humility and made him respect all life. (PROSE: Divided Loyalties) The Seventh Doctor implied that the Time Lords might have made a deal with them because of this power. (PROSE: No Future)

The Guardians could act together to directly bend reality and alter space and time, as when they retroactively un-did the very existence of Prometheus for violating the Ancient Covenants. They essentially were the universe, and so they would not dare interfere; they could never be seen to act in things or be involved so as to preserve the structure of reality, (PROSE: The Quantum Archangel, TV: Mawdryn Undead) bound not by physical laws but rather codes of conduct as were the Chronovores and possibly other transcendental beings. (PROSE: No Future)

Since they could not be seen to act directly, and because they were evenly matched and needed someone to give them an edge, the Black and White Guardians operated through agents. However, while the agents had to make their own decisions and agree to help, the Guardians created a web of choices to influence the agents decisions. (PROSE: The Well-Mannered War) The Guardians were themselves agents used by the Grace. (AUDIO: The Judgement of Isskar, The Chaos Pool)

The White Guardian sent the Fourth Doctor and his companions on quests for the two Keys to Time (TV: The Ribos Operation, COMIC: Time & Time Again), while the Black Guardian assigned the Shadow to stop him (TV: The Armageddon Factor).

The Black Guardian employed another agent, Turlough, (TV Mawdryn Undead) to kill the Fifth Doctor and later, with the White Guardian, sponsored a contest for a group of Eternals to win Enlightenment. (TV: Enlightenment) The Crystal Guardian, however, simply pursued private games. (PROSE: Divided Loyalties)

Behind the scenes

 * The existence of the Celestial Toymaker as a character on Doctor Who pre-dates the introduction of the Guardian mythos. Later continuity retroactively made him a Guardian, and still later continuity made him an Elder God, where the Guardians are within the Elder Gods. Hecuba (called the Queen of Time) was introduced as his sister, so whether she was a Guardian or how she was connected to them or their role was not specified.
 * Until Divided Loyalties and The Quantum Archangel, reference to the Guardians only consisted of Black and White, and to date, they are the only Guardians to be seen, depending on what the Celestial Toymaker really is.