The Infernus Occulum is a book written by Averroes, the Prophet. It played heavily in the birth of the Goddess Keresis and the Twin Lords of Evil, Shaitan and Apollyon. An exceprt from the book reads:
...The Infernus Occulum, translated variously as the Gate of Hell or the Eye of Hell, is the true source of the red mists, and of Sartan's disfigurement. Only a vastly complicated ritual opens the Gate, and its details are known only by a few.