Azathoth (All-Consuming Fire)

A slug-like telepathic entity who answered to the names of "Azathoth" or "Azazoth" was imprisoned on the planet of Ry'leh. It used its hypnotic powers to control its followers and could convert those followers into rakshassi.

Biography
To her followers, Azathoth claimed that she, the original Great Old One Azathoth, had spent aeons in the void after giving birth to the universe, but, hearing the prayers of beings who worshipped her, had finally managed to take on physical form. Her attempts to use the Word on all who did not believe in her ended with her imprisonment on the ice planet Ry'leh, with a garrison of Shlangii guarding her at all times. She said that she had let them do this because, in her infinite benevolence, she did not wish to have to kill these non-believers, even as it cost her her freedom.

The Doctor, however, refused to believe this Azathoth was the real deal; to him, it was but a mockery, making its living by usurping the reputation of the real entity using its mild telepathic ability to control the faith of lesser beings. Sherringford Holmes violently disagreed, stating that this theory was only the Doctor grasping at straws to explain the inexplicable.

Whatever it was, the imprisoned physical Azathoth tried to escape its containment and establish a new cult on Earth, but was eventually transported by the Doctor to 1906 San Francisco, where its body died (along with all the rakshassi she had created) 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 for the audio adaptation of All-Consuming Fire.