Cthulhu

Cthulhu was, depending on various sources, either a fictional creature created by H. P. Lovecraft, or a Great Old One that the Seventh Doctor encountered in Haiti.

As a fictional being
The Somnifax was attracted to H. P. Lovecraft and entered his mind in an attempt to bring Cthulhu into reality. The Sixth Doctor and Flip Jackson encountered him when they were in Lovecraft's mind. (AUDIO: The Lovecraft Invasion)

As a real creature
One account identified as Cthulhu (PROSE: All-Consuming Fire) an "Old One" whom the Seventh Doctor prevented from rising again in 1915 Haiti. (PROSE: White Darkness) According to the account which identified him as Cthulhu, he was one of the "Great Old Ones" in the sense of a group of beings from a universe which existed before the Big Bang, who had escaped the destruction of their universe by shedding their physical forms, and had been reborn in the new universe with godlike powers. This description tallied (PROSE: All-Consuming Fire) with another acount of the "elder gods", which specified that these beings had been the Time Lords of their universe but did not name Cthulhu as one of them. (PROSE: Millennial Rites) A further account listed Cthulhu as one of these "Great Old Ones", survivors of the pre-universe and possibly its Time Lords, and stated that he had been named by Rassilon in an effort to give the future Time Lords power over these creatures. (PROSE: Divided Loyalties)

The account primarily dealing with the Seventh Doctor's adventure in Haiti, however, gave a somewhat different account of the "Great Ones", as a specific, not-necessarily-godlike beings who were not well-known to the Time Lords. According to this account, the Great Ones, later Great Old Ones, were a race who had crossed over into the Doctor's universe from their own, which was located on "the outer planes", some 15 million years before the 20th century. They had then taken over various planets, including a prehistoric Earth where they coexisted with the Silurians and the Star People.

Eventually, however, the Silurians went into hibernation and the Star People returned to the stars after their servitors rebelled against them. The Great Ones were besieged by powerful predators and decided to entomb their physical forms underground while separating their consciousnesses, which moved through the Time Vortex on the Time Winds. This particular Old One entombed itself in a stone crypt deep within the planet in the caverns underneath Hispaniola. The body retained a subconscious mind, and the two could not be merged until an alignment of the stars as they had been when the entity was last whole. Due to the universe's constant expansion and evolution, perfect alignment would be impossible and the Old One needed others to perform rituals that would give the subconscious additional focus in drawing the consciousness from the Vortex.

When the correct alignment occurred in 1915, the Old One's body was able to dream for several days and this created a pervasive psychic background at .338 microbars of pressure. Gilles Lemaitre, using the vodoun ritual of Wete Mo Nan Dlo, attempted to restore the Old One. The Old One, in turn, helped Mait in his plans to create an army of zombies and a device to control them. Before Mait could awaken the Old One, the Doctor managed to set explosives in the tomb which killed Mait and resealed the Great Old One's tomb. (PROSE: White Darkness)

Legacy
By the 21st century, Cthulhu's name entered popular culture. Cthulhu Gate was a software company that made computer games. (PROSE: Cat's Cradle: Warhead)

Origin Zero used Cthulhu as an example of a "dark god" when trying to guess at Morch'aliach's nature. (PROSE: A Honeycomb of Souls)

Behind the scenes

 * Cthulhu first appeared in H. P. Lovecraft's short story "The Call of Cthulhu".
 * The Great Old One in Haiti wasn't identified by name until the novel All-Consuming Fire, in which the Doctor associates a number of past enemies with specific Great Old Ones.
 * Cat's Cradle: Warhead predated later Virgin New Adventures which referenced elements as real in the Doctor Who universe.
 * R'lyeh, the city Lovecraft's Cthulhu was entombed in, is referenced as "Ry'leh", the planet in which the false Azathoth was imprisoned, (PROSE: All-Consuming Fire) as R'lyeh, a Mondasian creature, (COMIC: The Cybermen) and as a word that appears in a quote from the Necronomicon. (PROSE: The Banquo Legacy)
 * The Necronomicon quote from The Banquo Legacy is two parts, a translated couplet and a sentence in an unknown language. Both occur in "The Call of Cthulhu", the latter a psychic call made by Cthulhu itself which is translated in-story. The word Cthulhu itself was removed from the quote in The Banquo Legacy. All-Consuming Fire uses a similar set of words but indicates it is a meaningless polyglot mixture which uses discords to weaken spacetime.