Azathoth

Azathoth 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 was imprisoned on the planet of Ry'leh. It used its hypnotic powers to control its followers and converted those followers into Rakshassi. It and its followers died in a fire when the temporal rift connecting Ry'leh to Earth was diverted by the the Doctor to San Francisco in 1906, at the date of a great earthquake. (NA: All-Consuming Fire)

Behind the Scenes
Azathoth originated in the writings of H. P. Lovecraft where it is part of the upper echelon of the Great Old Ones who act on a vast cosmic scale and exist outside of our universe.

Likewise, Ry'leh is named after the underwater city which is referred to in some of his work.