“…the Necronomicon, a highly secret magical text released in paperback.” – William S. Burroughs.

This symbol, called the Necronomicon gate, or the”gate of Yog-Sohoth,” is a faux sigil usually found on the cover of an edition of the legendary and quite fake “Necronomicon,” a falsified grimoire based on the work of horror fiction writer H. P. Lovecraft.
For a more detailed look at this fabled work, see: Necronomicon, truth or fiction?

Necronomicon Gate