N'gah-Kthun is a name coined by H. P. Lovecraft for his novella The Whisperer in Darkness, in which it is heard as part of a fragmentary conversation between the Mi-Go and their allies.
The name appears with the variant spelling N'gha-Kthun in August Derleth's short story "The Return of Hastur", where it is part of a list of beings, places, and concepts that Paul Tuttle learned about while studying occult grimoires.
In "The Horror in the Gallery", by Lin Carter, N'gha-Kthun is at last identified as one of the Lesser Old Ones, and the leader of all the Mi-Go who serve Hastur.