Nyarlathotep

Nyarlathotep was one of the Great Old Ones given its name in the present universe by Rassilon. (PROSE: Divided Loyalties) Elektra considered him "the darkest and greatest of the Old Ones", and despised him for "what he had done"; she hoped her child, Kronos, would be the champion of a united universe against him and his kind. (PROSE: The Quantum Archangel)

The Satyrmen of El Mundo knew Desolador, who brought a destructive technological plague to their planet, as the "Scion of Nyarlathotep". (PROSE: The Rise & Fall of Señor 105)

The Sixth Doctor and Flip Jackson encountered a fictional version when they were in H. P. Lovecraft's mind brought to life by the Somnifax. (AUDIO: The Lovecraft Invasion) The Seventh Doctor told Bernice Summerfield that he sincerely hoped never to meet Nyarlathotep. (PROSE: All-Consuming Fire) Roz Forrester, 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".
 * In 2013, when asked how he would respond if presented with a theory that the Other was Nyarlathotep, Andrew Cartmel replied, "I would respond, "Who is Nyarlathotep?" while scratching my head."
 * Callum Phillpott once noted that the "man in black" in Phillpott's own Cyber-Hunt could, among other possibilities, be interpreted as Nyarlathotep.