The Mountains of Shadow (Ephel Dúath) were a virtually impassable "L"-shaped range that formed the western and southern boundaries of Mordor.

Only two passes through this range were ever discovered. In the North, where the Ash Mountains met up with the Mountains of Shadow, the pass known as Cirith Gorgor was found. This pass was controlled by Sauron's Black Gate and has been known ever since Sauron first occupied Mordor.

The second pass, Cirith Ungol, was found in the western arm of the mountains across from the eastern end of the White Mountains.

In the year 2000 of the Third Age a host from Mordor issued through Cirith Ungol to assail Minas Ithil, the city and tower erected by Gondor as a watch on Mordor. In March of TA 3019 Frodo and Samwise traversed Cirith Ungol to enter Mordor on their quest to destroy the One Ring.
Encyclopedia entry originally written by PotbellyHairyfoot