The dale of Udun was a large bowl-shaped valley located at the junction of the Ash Mountains and the Mountains of Shadow - Mordor.

Udun served Sauron as part of the main entrance to Mordor from the west. From the Dagorlad, to enter Mordor, one had to pass through theMorannon and travel across the gap of Cirith Gorgor to reach Udun. Upon crossing Udun one then traversed the pass of Isenmouthe through the gap of Carach Angrem to reach the Plateau of Gorgoroth and the interior of Mordor.

By the time of the War of the Ring, Udun was filled by the forces of Sauron with deep tunnels and armouries for the defence of the Morannon. The castle fortress of Durthang, built by Gondor on the western slopes of the valley at the start of the Third Age, had become only one of the many orc-holds to be found there.

After the destruction of the Ring, most of the holds and constructions in Udun crumbled and fell in ruin as the evil strength that held them vanished with the Ring into the fires of Mount Doom.
Encyclopedia entry originally written by PotbellyHairyfoot