The Necronomicon is a book in H. P. Lovecraft's short stories that supposedly contains accounts of the old ones and the means for summoning them. The Cthulhu cultists in South Park read it while they await Cthulhu's arrival. It features heavily as a plot point in "Mysterion Rises" and "Coon vs. Coon & Friends".
It is revealed in "Mysterion Rises" that Kenny McCormick (while under the guise of his alter-ego Mysterion) was affected the book. After hearing the line "That is not dead which can eternal lie, and with strange aeons even death may die," he feels troubled, and it is mentioned that his parents Stuart McCormick and Mrs. McCormick were present at cult meetings (though they were only there for the free beer).
The following episode, "Coon vs. Coon & Friends", implies heavily that the Necronomicon was involved in Kenny gaining his power of immortality - he notes the Nightmare City of R'Lyeh as familiar to him, and while he reads the book, there is a panel seen showing a spell being cast on a baby by cultists. This is further of note as Trey and Matt state in the audio commentary that a "panel" in the episode hints at Kenny's origins.
- Like Cthulhu, the Necronomicon is originally from science-fiction/horror writer H. P. Lovecraft's works.
- Henrietta mentions the book as having been written by a "mad prince". In H. P. Lovecraft's works, the book was written by a mad Arabic poet or scholar (not referred to as a prince) named Abdul Alhazred. The name "Prince Abdul Alhazred" actually appears briefly in "Coon vs. Coon & Friends", alongside an illustration, when Henrietta shows Mysterion the book.
- The line "That is not dead which can eternal lie, and with strange aeons even death may die," is also said to have been a line from the book in Lovecraft's original story, "The Call of Cthulhu".