Udûn was a wide, depressed valley in northwestern Mordor that lay between Cirith Gorgor and the Carach Angren.
Description
Probably the origin of Udûn is related to the volcanic activities present lands of Mordor. The Atlas of Middle-earth by Karen Wynn Fonstad suggests that the genesis of Udûn was linked to the formation of Mount Doom. In fact, according to Fonstad, the valley could be a caldera or more likely an anular dike.
History
When Sauron settled in Mordor he exploited the geography of Udûn in order to make it the pivot of his defense against enemies, sealing Cirith Gorgor with the Black Gate and building forts at the Isenmouthe. Within its circumference were the forges of Sauron's armies and many Orc-holds, including Durthang.
During both the War of the Last Alliance and the War of the Ring, the vast armies of Mordor under the command of Sauron passed through here and out the Black Gate to confront their enemies. With the destruction of the One Ring, Udûn was changed, and the Black Gate thrown down.
Etymology
Udûn is a Sindarin word meaning 'Hell'.
In adaptations
The Lord of the Ringsfilm trilogy
Udûn appears in the third film of Peter Jackson's The Lord of the Rings film trilogy, while Frodo Baggins and Samwise Gamgee are traveling towards Mount Doom. The valley itself is depicted similarly to its description in J.R.R. Tolkien's works, though the apparent distances are altered in order for Barad-dûr to be visible from the Black Gate.
InMiddle-earth: Shadow of Mordor
In Middle-earth: Shadow of Mordor, following the Battle of Dagorlad, Udûn became populated by the Outcasts of Udûn, descendants of banished Gondorian criminals and slaves. They built a shanty town known as The Slabs in the eastern reaches of the valley. Upon Sauron's return to Mordor, the Outcasts were either enslaved or driven out of Udûn and their city was converted into Uruk's Hollow, a slave camp and home for Sauron's minions.
The Outcasts had an adversarial relationship with the Rangers of the Black Gate under Mordor, but with Sauron's return the free Outcasts rallied under the Ranger deserter Hirgon, making their headquarters in a series of Barrows that the Orcs kept away from out of superstition. Many Outcast slaves were liberated by Talion as he campaigned in Udun, but later they attempted to escape from Mordor, with their ultimate fate unknown.