Cthulhu Mythos in general.
H.P. Lovecraft created the Mythos, to which Nyarlathotep belongs, in his short stories that were, primarily, published by Weird Tales magazine during the pulp era. My first exposure to Lovecraft was in an episode of the Real Ghostbuster's entitled the Collect Call of Cthulhu, I was apparently 15. However, it wasn't until his writings were mentioned in a Fortunato short story by Lewis Shiner in the 1st or 2nd volume of the Wild Cards anthology that I began to seek out Lovecraft's work.
I like my cosmic horror more action-packed and less hopeless than H.P.'s stories, but the roots he planted are essential to the types of worlds I like to grow. What better antagonists than fungi from a distant world, unseen hunters that amush us from the angles of time, or amoeboid monstrosities that leave their victims gibbering lunatics? Who better for a charismatic cult-leader to venerate than an uncaring toad god? And what setting doesn't need a vile tome written by a madman to ensure the awakening of a dreaming horror when the Stars are Right?
Lovecraft's Mythos shambles its way into adventures I'm planning from non euclidean recesses deep inside my brain. After all, why should the tall, smiling stranger, who your party encounters in a tavern and who happens to know the location of some demoniac temple, have your best interests at heart?