Choronzon is a demon or devil that originated in writing with the 16th century occultists Edward Kelley and John Dee within the latter's occult system of Enochian magic. In the 20th century he became an important element within the mystical system of Thelema, founded by Aleister Crowley, where he is the Dweller in the Abyss, believed to be the last great obstacle between the adept and enlightenment. Thelemites believe that if he is met with proper preparation, then his function is to destroy the ego, which allows the adept to move beyond the Abyss of occult cosmology.
(extract source - http://en.wikipedia.org/wiki/Choronzon )
More info: http://www.thelemapedia.org/index.php/Choronzon - http://www.ocultura.org.br/index.php/Choronzon