Drawing a deep breath they passed inside. In a few steps they were in utter and impenetrable dark. Here the air was still, stagnant, heavy, and sound fell dead. They walked as it were in a black vapour wrought of veritable darkness itself that, as it was breathed, brought blindness not only to the eyes but to the mind, so that even the memory of colours and of forms and of any light faded out of thought. Night always had been, and always would be, and night was all. -- Tolkien, J.R.R. The Two Towers. New York: Ballantine Books, 1955.

Cirith Ungol is a pass located high in the Mountains of Shadow on the western border of Mordor.  Pronounced Keereeth Oongoll, the name is Sindarin for Spider's Cleft, or Pass of the Spider.  It is the only way into the land from the West and is guarded by the Tower of Cirith Ungol.  Although the tower was originally built by the Men of Gondor after the War of the Last Alliance of Elves and Men was fought, it is now occupied by the evil forces of Sauron.  The pass is also guarded by Shelob, an ancient and foul creature that haunts a network of tunnels at the pass of Cirith Ungol and preys upon unsuspecting travelers.