Azathoth

Azathoth, or Azazoth, was the weakest of the Great Old Ones, worshiped by the Silurians and Shobogans as a god of chaos and anarchy. (PROSE: All-Consuming Fire)

In some myths, Azathoth was the personification of the force from which the universe sprang, a singularity that grew and grew until it was unable to sustain itself as a single entity, and so divided and split. The Fourth Doctor described the universe as not being separate from the singularity, but becoming different and forgetting where it came from. (PROSE: Tomb of Valdemar)

Sherlock Holmes and the Seventh Doctor encountered a slug-like creature calling itself Azathoth, decried by the Doctor as a mockery making its living by usurping the reputation of the real entity. It tried to escape its imprisonment on the planet of Ry'leh, and was eventually transported by the Doctor to 1906 San Francisco, where it died in a fire following an earthquake. (PROSE: All-Consuming Fire, AUDIO: All-Consuming Fire)

Behind the scenes

 * Azathoth originated in the writings of H. P. Lovecraft where it is the greatest of the Great Old Ones, inconceivably huge and powerful entities who act on a vast cosmic scale and exist outside of our universe (sometimes interacting with it to catastrophic effect). Azathoth in particular is the most monstrous and powerful, a mindless demiurge eternally dreaming at the centre of the Universe, which is in itself its own dream. His awakening is a cosmic-level event, heralding the end of all things.
 * The name was changed to Azazoth in the audio adaptation of All-Consuming Fire.