Azathoth

Azathoth, or Azazoth, was tone of the Great Old Ones, worshipped by the Silurians and Shobogans as a god of chaos and anarchy. She was said, in many of the old legends, to have been the weakest of the Great Old Ones. The Seventh Doctor described Azathoth as "an amorphous blight of nethermost confusion that blasphemes and bubbles at the centre of all infinity, coexistent with all Time and conterminous with all Space". (PROSE: All-Consuming Fire)

Biography
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 purple, telepathic slug-like entity calling herself Azathoth, who had the ability to brainwash people into serving her unconditionally by speaking what was only referred to as "the Word". She could also bestow the Mark of Azathoth upon them, transubstantuating them into rakshassi.

To her followers, Azathoth claimed that she, the original Great Old One, had spent eons 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 thing; 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 was the greatest of the Outer Gods. Unlike other Outer Gods, like Nyarlathotep and Yog-Sothoth, Azathoth was unaware and non-sapient, often called the Blind Idiot God.
 * The name was changed to Azazoth in the audio adaptation of All-Consuming Fire.
 * In All-Consuming Fire, the possibly-false Azathoth and her followers refer to her as a "she". John Watson, on the other hand, always uses the pronoun "it" when talking about the creature.