Shub-Niggurath

Shub-Niggurath was an inhabitant of the previous universe named by Rassilon. (PROSE: Divided Loyalties) When that universe came to an end, she and others of her kind entered this universe, where they discovered that they were virtually omnipotent. Shub-Niggurath, who was pregnant at the time, gave birth on Polymos, thus giving rise to the Nestene Consciousness. The Sixth Doctor suggested that she died (PROSE: Synthespians™) but later cited her success conquering Polymos with her offspring as embarrassing her peer, Yog-Sothoth. (PROSE: Millennial Rites)

A group of sentient TARDISes from the War in Heaven shouted "Ia Shub Niggurath!" whilst speaking with their little sister, Compassion. (PROSE: The Taking of Planet 5)

Behind the scenes
Shub-Niggurath was an outer god created by H. P. Lovecraft. Worshipped on Earth and Yuggoth, her cult is apparently connected with fecundity.

In his notes for The Quantum Archangel, Craig Hinton elaborated on his concept of the wider cosmology of the Doctor Who universe, including the idea of the Great Old Ones as the survivors of an earlier race of Time Lords from the previous universe. Therein, he identifed Shub-Niggurath as having been "that universe’s equivalent of the head of the CIA".

Shub-Niggurath