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, elemental forces embodying several 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
 * The Rainbow Guardian of the Quantum Realm

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 fashion space and 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. (TV: Enlightenment)

The Guardians' natural form, like the other Old Ones, was a collective consciousness possessing neither form nor substance and existing between dimensions. (PROSE: Divided Loyalties) They could assume a form for a brief period as an "interface" with the universe, though they found it "cramped" existing in five dimensions. (AUDIO: The Destroyer of Delights) According to one account, the Black and White Guardians were initially one being, created by another Guardian before splitting upon physically entering the universe. (PROSE: Power to the People)

Powers
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) and stating that they were capable of altering time on a grand scale; the power they represented could not exist on something as small as a planet. (PROSE: Blood Heat, COMIC: Time & Time Again)

In relation to their omnipotence, 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)
 * Channelling 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)
 * Inserting themselves into the time stream (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)
 * Additionally, the Black Guardian threatened Turlough with immortality, but this may have been a lie. (TV: Enlightenment)

In terms of weaknesses, the Guardians could not reach into closed super-dimensional spaces. Because their being was linked to the Key to Time, if the Key decayed they would become affected, susceptible to energy weapons and age. (AUDIO: The Destroyer of Delights) However, while they were able to use the Key to Time, they were not left with an imprint of it as was the case with others. (PROSE: Power to the People) Additionally, for their seemingly omnipotent power, the Eighth Doctor claimed even they couldn't stop the universe from expanding. (PROSE: Dominion)

The Black Guardian was affected when the TARDIS's defensive systems were fully activated, reshaping into his negative form for unexplained reasons. He was also 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) and contact with Enlightenment. (TV: 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 represented the totality of the Guardians' power, 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 lest they become stale. (PROSE: Divided Loyalties)

Like the other Old Ones, the Guardians originated in another universe, and Rassilon himself considered the Old Ones "sub-Guardians." He asked questions of the "Guardians of the Universe," but 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 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)

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) They considered the Eternals and Chronovores to be their "own children." (PROSE: The Quantum Archangel)

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 their decisions. (PROSE: The Well-Mannered War) When the White Guardian told the Black Guardian he would never destroy the light, the Black Guardian responded that others would do it for him. When he retorted that the White Guardian's powers were fading, the other responded that others would recharge them for him. (TV: Enlightenment) The Guardians themselves were used as agents by the Grace. (AUDIO: The Judgement of Isskar, The Chaos Pool)

The White Guardian sent the Fourth Doctor and his companions on two quests for the Key to Time, (TV: The Ribos Operation, COMIC: Time & Time Again) while the Black Guardian assigned the Shadow to stop him. (TV: The Armageddon Factor) He later employed another agent, Turlough, (TV: Mawdryn Undead) to kill the Fifth Doctor. Later, with the White Guardian, he sponsored a contest for a group of Eternals to win Enlightenment, transferring his power into Wrack to help her win. (TV: Enlightenment) The Crystal Guardian, however, simply pursued private games. (PROSE: Divided Loyalties)

Other references
Zellin, self-identified immortal god and partner of Rakaya, was aware of the Guardians and their "power struggles". While trapping the Thirteenth Doctor aboard a space platform, Zellin told the Doctor that her dimension was like a board game for him of which "the Toymaker would approve". (TV: Can You Hear Me?)

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, despite the original intent being that he was an especially powerful member of the Doctor's own species, and still later continuity making him an Elder God — though the latter idea was reconciled with the earlier lore by the positing of the Guardians as being a subgroup among 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.