The Necronomicon is a forbidden book holding rites to call and command the Great Old Ones and truths of the cosmos that drive its readers to madness. Its original title is "Al Azif," said to be written by the mad poet Abdul Alhazred.
Though wholly invented by Lovecraft, it was cited so precisely that people kept believing it real. The way the book circulates as if it exists is itself a sign of the Mythos’s self-propagation.