Great Old One

The Great Old Ones, sometimes known simply as the Old Ones, were beings that existed before the universe. More a pantheon than a species, the Old Ones included the Elder Gods, Toymakers, Guardians of Time and even "Greater Old Ones" the Grace.

Overview
The Great Old Ones were unimaginably ancient creatures that were preserved in humanity's race memory as their worst nightmares. They were conceived as creatures that spread evil, chaos and destruction wherever they went. (AUDIO: The Roof of the World)

Quantum mnemonics was known to be the language of the Old Ones, as they represented physical laws of the Universe that preceded the current one. It was considered the Old Ones' equivalent of Block Transfer Computations and allowed them to rewrite reality on the most fundamental of levels. (PROSE: Millennial Rites) It was known that the Great Old Ones were incapable of being killed in the normal dimension. They also knew of the Time Lords and considered them a threat. (PROSE: White Darkness)

Biology and power
The Old Ones originally possessed physical bodies in their own universe. Upon entering N-Space, they received new bodies that may or may not have had heads, (PROSE: Millennial Rites) and some lost theirs when they ran from the Fendahl, abandoning their forms and letting their minds drift into the Time Vortex. (PROSE: White Darkness) Afterwards, they could assume a form for a brief period but not forever, their new natural state being a collective consciousness, possessing neither form nor substance. They existed between dimensions where they created universes, planes, and whatever else suited them, (PROSE: Divided Loyalties) including the Land of Fiction (PROSE: Conundrum) and the Celestial Toyroom, which the Toymaker created and resided in, able to step outside but never fully leave. (AUDIO: The Magic Mousetrap)

They sought to return to their bodies and had a third part of their form which existed in the outer dimensional planes which some believed was where they originated from. They could not be killed in the normal dimension, (PROSE: White Darkness) the Toymaker explaining that were he to die he would simply reappear. (AUDIO: Solitaire) They couldn't die as the energies their bodies were composed of were not meant to exist in the normal universe, and were they to decay, the released energy would rip apart space and time. They could only safely die in their own universe, or the Board, which caused the Time Vortex to turn inside out. (AUDIO: Signs and Wonders)

The sensors of a TARDIS noted the body of an Old One appeared as a massive quantity of energy which was the equivalent of twenty billion nuclear bombs. (PROSE: Millennial Rites)

While they were considered an actual race, (PROSE: Twilight of the Gods) their biology differed between individuals. The Celestial Toymaker carried his own mass with him, projected from his original universe. (AUDIO: The Nightmare Fair) He and the Gods of Ragnarok maintained humanoid appearances, as did the Guardians, although Rassilon theorised that they took on familiar forms to whoever they appeared before. (PROSE: Divided Loyalties) The Animus, Intelligence, and Fenric were noted for having multi-limbed bodies, a form the Nestenes generally took; although an Old One, the Nestenes were their own race, albeit a "singular entity" because of their hive mind.

The powers of the Old Ones varied between individuals, but included time travel or following someone in time, (TV: The Web of Fear, The Curse of Fenric, PROSE: The Well-Mannered War) existing in multiple time zones, (TV: The Greatest Show in the Galaxy) possession, (TV: The Abominable Snowmen, The Web of Fear, The Curse of Fenric) creating thunder and lightning, (TV: The Greatest Show in the Galaxy, The Curse of Fenric) re-animating dead bodies, (TV: The Greatest Show in the Galaxy) generating fog/fungus and webbing, (TV: The Web Planet, The Web of Fear) stopping a TARDIS in flight, (TV: The Web of Fear) extending an individual's life, (TV: The Abominable Snowmen, PROSE: White Darkness, AUDIO: The Nightmare Fair) and manipulating and relocating timelines. (AUDIO: Protect and Survive)

Society
Before arriving in N-Space, the Old Ones were thought to be the equivalent of Time Lords in their universe. (PROSE: Millennial Rites) While many Old Ones were imprisoned or located elsewhere, a group known as the Elder Gods occasionally came together, usually to watch each other play games on the Board. Despite their great power, they needed the lesser creatures of the universe to give them perspective in spite of their hatred for them. To them, the games were all that mattered to make existence worthwhile. Among themselves, it was the general consensus that Fenric was too dangerous, even by their standards. (AUDIO: Gods and Monsters) They had previously banded together to lock the Eidolon away in another dimension, fearful of its ambition. (AUDIO: Beyond the Ultimate Adventure)

Despite also playing games, some of the Elder Gods thought of the Eternals as leading pathetic existences. Within the Old One pantheon were the Guardians, (PROSE: Divided Loyalties) the Grace, (AUDIO: The Chaos Pool) the Toymakers, and reference has been made to the survivors of the pre-universe, including "a handful of baby godlings and Great Intelligences." (PROSE: Christmas on a Rational Planet)

History
According to the Time Lords, the beings that became known as the Old Ones did not originate from this universe, but the one before it. They were originally a powerful race of beings who were the equivalent of the Time Lords in their native space. However, when their Universe ended, the Old Ones shunted themselves into a parallel dimension which collapsed seconds later into the new Universe. This allowed them to pass into the new Universe which had different laws of physics, and they discovered that they had gained god-like powers which allowed them to manipulate the new Universe in which they now existed. (PROSE: Millennial Rites)

However, the Carnival Queen maintained that the Great Old Ones were not from a previous universe, but from an early period in this universe, before the Time Lords imposed reason and order upon it. (PROSE: Christmas on a Rational Planet)

The Great Old Ones ruled the galaxy (presumably the Mutter's Spiral), eventually only being left in the memory of half a dozen worlds. (PROSE: The Nameless City) They were known throughout the universe as terrible creatures that sought to destroy others or incorporate them into their being. (PROSE: Twilight of the Gods) They were worshipped as gods on Earth by the Silurians and even the gargantuan entities that ruled Earth before them; they were also known to a cult of the Shobogans. According to the legends, it was believed that Azathoth was the weakest of their kind. (PROSE: All-Consuming Fire)

Ancient carvings dating at least fifteen million years ago showed that the Old Ones ruled everywhere before they died out and went into hibernation after fleeing from the "predators with a touch of decay". They managed to escape from the great cosmic disaster in order to flee into the Time Vortex. There they became discorporate entities, thus leaving their bodies behind on Earth. Their minds sought to return to their bodies when the stars were correctly aligned with the tidal forces of the stellar masses, an event which could bring about a sufficient tear in the space-time continuum to create a pathway back to their bodies. However, as the stars were constantly expanding and contracting, they never returned to the same configuration as the time when the Old Ones went into hibernation. Thus, they required the assistance of their followers to attract the attention of their bodies; autonomic instincts and allow them a chance to compensate. As they were unable to affect the world physically, they used the influence of their powerful minds to achieve their goals. (PROSE: White Darkness)

The Pharaoh Amenhotep II discovered them at one time on Earth and attempted to harness their power for himself. However, he came to realise that they were beyond his control and lured them into a white pyramid with a structure of infinite reflections where the creature's evil was to be trapped forever. This trap succeeded and kept the world safe from their evil. (AUDIO: The Roof of the World)

By the 2020s, when To'Koth had reached the end of dying of old age over billions of years, the Seventh Doctor took him back to the Board to die, humbling and terrifying the other Elder Gods as he was the first God to die. As a way of ending hostilities, To'Koth called a truce between the Gods and the Doctor. (AUDIO: Signs and Wonders)

Other references
According to one account, in 1936 human writer H. P. Lovecraft wrote about the “Old One, a.k.a. Elder Thing" in At the Mountains of Madness, which was "widely regarded as real by 25th century fringe archaeologists". The Elder Things would have been brought to life by a fictional generator. Compassion joked with the Eighth Doctor about him being a Great Old One on his mother's side. (PROSE: The Taking of Planet 5)

List of Great Old Ones
Amongst those listed as members of the Old Ones were Azathoth, Cthulhu, Dagon, Lloigor, Yog Sothoth, Nyarlathotep, Shub-Niggurath and its offspring the Nestene Consciousness, Kwundaar, Traguam, Tor-Gasukk, Melefescent, the Gomagog (including Gog and Magog), Eidolon, the Archons and the Grace.

An offshoot of Old Ones were known as the Elder Gods, who shared powers almost identical to that of Old Ones but were arguably more direct in their meddling with the universe and their obsession with games on the Board. (AUDIO: Gods and Monsters) Members of this group included the Great Intelligence, Hastur the Unspeakable (known as Fenric), Weyland, the Lloigor known as the Animus, the Mi'en Kalarash, Derleth, the Karnas'koi (including CP Doveday), the Kai'lizakia, Moloch and his servants (disguised as Albert and Peggy Marsden), the Gods of Ragnarok (including Raag, Nah and Rok and the God of Ragnarok), To'Koth and Ginny Greenteeth. A sub-group of Elder Gods were the Toymakers, which included the Celestial Toymaker and his sister Hecuba.

Far above both groups were the Guardians of Time, considered masters of reality, and the upper echelons of the Great Old Ones. Rassilon even considered the Old Ones "sub-Guardians". (PROSE: Divided Loyalties) They represented elemental forces embodying several aspects of the universe, with the two most prominent being the White Guardian (representing Light and Order/Structure) and the Black Guardian (Darkness and Chaos/Entropy). Other Guardians included the Crystal Guardian (Dream and Fantasy), the Red Guardian (Justice and Truth), the Azure Guardian and its twin (Equilibrium and Balance) and the Gold Guardian (Life and Death).

Above even the Guardians were the Grace, who worked to hold the universe together and occasionally stepped in to stop reality breaking apart. They created the Key to Time and used the Black and White Guardians as agents, and could send the Guardians back into the echoing void if they wanted. (AUDIO: The Judgement of Isskar, The Chaos Pool) They were presumably the "greater Old Ones" that the Guardians would refer to at times, such as the imprisonment of the Chronovore Kronos within his crystal and into the Time Vortex. (PROSE: The Quantum Archangel)

In addition, an unnamed Great Old One was said to be within the Infinite as a power source. (TV: The Infinite Quest)

Behind the scenes

 * The Great Old Ones originated out of the, a based on the writing of H. P. Lovecraft and to an extent, some of his contemporaries with which he had corresponded or made extensive revisions for. A few characters and concepts in televised Doctor Who, such as Sutekh (though partly based on an actual ancient Egyptian god, known more commonly as Seth) and the Fendahl, show the influence of the Mythos, and the latter has similarities with Brian Lumley's . Not until the novels White Darkness, All-Consuming Fire and Millennial Rites did beings explicitly referred to as Great Old Ones appear in fiction set in the Doctor Who universe.
 * The latter two books took known Doctor Who enemies, such as the Animus, and retconned them into having links to the Mythos. The Taking of Planet 5 revealed that the Elder Things also existed in the form of fiction in the Doctor Who Universe. Though the established Cthulhu Mythos and the Doctor Who version have some differences, stories set in the Mythos frequently contradict themselves.