Udûn

Behind the Black Gate, in the angle where the Ephel Dúath and the Ered Lithui came together, lay a great hollow among the mountains, the valley of Udûn. Its floor was pitted with the holes and tunnels of the Orcs, and its cliffs were honeycombed with caves, forges and barracks in which the garrisons of the gate and the armies bound for the north were housed. At its south-eastern end the mountains drew together again in a second pass, the Isenmouthe, or Carach Angren, which was fortified with a wall, a ditch and a gate of its own, so that an enemy who broke through the Morannon would find a second fortress waiting. From the Isenmouthe a road ran south-east across the plateau of Gorgoroth to Barad-dûr. On the western heights above the valley stood Durthang, an old fortress of Gondor turned into an Orc-hold, from which a road came down to the Isenmouthe.

The name Udûn was an old Elvish word for Hell, and the same name had been used by the Elves for Morgoth's fortress of Utumno.

Durthang was built by Gondor in the early Third Age as one of the forts watching Mordor and was abandoned with the rest after the Great Plague. When Sauron returned Udûn became the northern arsenal of his realm. In March 3019 its holds emptied as the armies marched to the Morannon. On March 18 Frodo and Sam, making their way north along the Morgai, were overtaken by a company of Orcs coming down from Durthang and forced to march with it. That night they were driven through the Isenmouthe with several converging columns, and in the crush at the gate they slipped away into the darkness and began the crossing of Gorgoroth. A week later, on March 25, the valley's remaining garrison joined the host that surrounded the Host of the West before the Black Gate and perished with it in the fall of Sauron.

Udûn lay at the north-western corner of Mordor, between the Morannon on the north-west and the Isenmouthe on the south-east, walled by the ends of both mountain ranges. Gorgoroth opened beyond the Isenmouthe; Mount Doom stood some forty miles to the south-east.