Azathoth

Azathoth, or Azazoth was the weakest of the Great Old Ones. The Silurians and Shobogans worshipped it as a god of chaos and anarchy.

A slug-like creature calling itself Azathoth, decried by the Seventh Doctor as a mockery making a living by usurping the reputation of said entity, was encountered by the Doctor and Sherlock Holmes on the planet of Ry'leh. (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.
 * Likewise, R'lyeh is named after the underwater city in which vaults Cthulhu, one of the few earthbound Great Old Ones, sleeps. The city is referred to in some of his work.
 * The name was changed to Azazoth in the audio adaptation of All-Consuming Fire.