Udûn (Mordor)

Udûn also known as Hell was a depressed plain in Mordor.

It lies between Cirith Gorgor and the Isenmouthe and was traversed by large armies of Sauron in times of war.

History
Udûn originates with the creation of Mordor which was probably the result of volcanic activity and tectonic movement that came with the tumultuous upheaval at the end of the First Age, and by the look of the site it many be a dead volcano.

During both the war of Last Alliance and the War of the Ring, the vast armies of Mordor passed through here and out the Black Gate to confront their enemies.