Yog-Sothoth

Yog-Sothoth was a Great Old One whom the Seventh Doctor said he'd "met in Tibet and again in London". (PROSE: All-Consuming Fire)

According to one account, in his original form, Yog-Sothoth was a member, and the military strategist, of a race of beings called the Great Old Ones, who were the equivalent of the Time Lords in a previous universe to the one the Doctor resided. They shunted themselves into a parallel universe to pass into the next universe. Yog-Sothoth discovered it had gained god-like powers and decided to try the various gambits and games it had only played on computers. Over the billennia, it mounted millions of campaigns against inhabited planets. It used the Hisk version of koalas on Hiskith and domestic animals equivalent to dogs on Danos.

This account held that Yog-Sothoth was the same entity as that known to the Doctor as the Great Intelligence, (PROSE: Millennial Rites) whom another account confirmed to be a Great Old One. (AUDIO: The Roof of the World) However, other, contradictory accounts of the Intelligence's nature and origin existed. (PROSE: Doctor Who and the Web of Fear, The Forgotten Son, Legacies, Night of the Intelligence)

Yog-Sothoth was one of the fictional characters that Missy had to defeat in order to take over the Land of Fiction. (PROSE: The Wonderful Doctor of Oz)

Behind the scenes

 * Yog-Sothoth is one of the Great Old Ones from H. P. Lovecraft's public-domain Cthulhu Mythos, which All-Consuming Fire brought into the fold of the Doctor Who universe. Second only in power to Azathoth among the Great Old Ones, Lovecraft's Yog-Sothoth often appeared as a mass of spheres.
 * Although only Millennial Rites was able to confirm this link, the implication that Yog-Sothoth and the Great Intelligence were one was already present in All-Consuming Fire, due to the locations in which the Doctor says he battled Yog-Sothoth. However, from a legal standpoint, it does not appear that the Great Intelligence can be said to feature in All-Consuming Fire, which features no copyright credit to the Intelligence's creators Mervyn Haisman and Henry Lincoln (as indeed it wouldn't have to for mentioning the public-domain Yog-Sothoth and very faintly implying a connection to an old Doctor Who character).