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
Cthulhu, (PROSE: All-Consuming Fire) like a number of other Great Old Ones, had been on Earth at some point following the Silurians going into hibernation. Centuries of warfare had damaged the planet and Cthulhu's only recourse was to entomb itself in a stone crypt deep within the planet in the caverns underneath Hispaniola. Its separated consciousness travelled along the time winds while its sleeping body retained its subconscious mind. The two could not be merged until an alignment of the stars as they had been when Cthulhu was whole. Due to the universe's constant expansion and evolution, perfect alignment would be impossible and Cthulhu needed others to perform rituals that would give the subconscious additional focus in drawing the consciousness from the Time Vortex.

When the correct alignment occurred in 1915, Cthulhu'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 Cthulhu. Cthulhu, in turn, helped Mait in his plans to create an army of zombies and a device to control them. Before Mait could awaken Cthulhu, 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.