Nyarlathotep

Nyarlathotep was one of the Great Old Ones given its name in the present universe by Rassilon. (PROSE: Divided Loyalties) He was considered the darkest and greatest of the Old Ones. Elektra, who despised Nyarlathotep most of all, hoped her child, Kronos, would be the champion of a united universe against him and his kind. (PROSE: The Quantum Archangel) The Seventh Doctor told Bernice Summerfield that he sincerely hoped never to meet Nyarlathotep. (PROSE: All-Consuming Fire) Roz, using her knowledge of Lovecraft's writing, managed to convince an agent of the Great Old Ones that the Seventh Doctor was their dark messenger Nyarlathotep himself. (PROSE: The Death of Art)"The Doctor is the emissary of your masters, worm. He is the messenger of the powers you draw upon. [...] He is more than man. He is the troubler at the gate, the walker with the thousand forms; the part of the idiot god which embraces the pain of reason. He is the thing from the void that mocks the blind apes of truth. He is the dark messenger, Nya..."

- Roz

Behind the scenes

 * Nyarlathotep originated in the horror stories of H. P. Lovecraft. In the Call of Cthulhu tabletop role-playing game Nyarlathotep is classified as an "Outer God", a class of beings considered even more powerful than the "Great Old Ones".
 * When asked if The Other could be Nyarlathotep, Andrew Cartmel admitted not knowing anything about a "Nyarlathotep"