Eibon is the supreme sorcerer of Hyperborea in Clark Ashton Smith's cycle, a devotee of Tsathoggua and author of the vast grimoire known as the "Book of Eibon." Within the broader Mythos the Book of Eibon ranks alongside the "Necronomicon" as one of the most dreaded forbidden texts.
Legend holds that Eibon escaped persecution by inquisitors through Tsathoggua's intercession, fleeing to the planet Saturn. Smith portrayed him in the ornate, decadent register characteristic of his Hyperborean tales, and both Lovecraft and Howard cross-referenced Eibon's works, cementing the shared-mythology framework.