In Udûn the Dark Lord was able to muster vast armies
This natural vale lay just within the Black Gate of Mordor
and it was here that Sauron was able to assemble in secret, large forces of Men and Orcs swayed by the dark power, until
such time as he was ready to unleash them upon the forces of the West.
This page written and edited by our Dark Historian Grievous
The vale of Udûn was created at the end of the First Age in the volcanic activity that shaped the rest of Mordor. For the first thousand years of the Second Age it lay empty, a depressed valley of barren granite and ash. Udûn was mostly lifeless, save for the occasional patch of withered moss and thorns. But in SA 1000, Mordor was occupied by the Dark Lord Sauron. In Gorgoroth Barad-dûr was built, and Orodruin, the Mountain of Fire, burned constantly. Sauron mustered great armies of Orcs, Wargs, Trolls, and Dark Men, and housed them in Udûn.
Roads were made between the Black Gate that barred the pass of Cirith Gorgor and the Isenmouthe, the gap between the mountain-spurs that lay at the southern end of the vale. Between them great camps were made. Small towers and warg-pits were built, and caverns were dug in the mountain-walls surrounding the vale to house countless mines, pits, dens, and armories. The roads received much traffic of messengers passing between Barad-dûr and the cities of allies in the east and south. It was in Udûn that the Orc-armies of Sauron gathered in preparation for the War of Sauron and the Elves.
The Last Alliance marched through Udûn on their way to Barad-dûr after the Battle of Dagorlad. During this period they burned most of the camps, buried most of the caverns, and destroyed most of the supplies housed in the vale. The Orcs fled to Gorgoroth, and the host pursued them. Udûn was once again vacant, and the Black Gate fell into ruin. The men of Gondor built the Towers of the Teeth to guard Cirith Gorgor, and Durthang to oversee the rest of Udûn. These watchfortresses were manned, for a while; but eventually they were abandoned.
When the Nazgûl returned to Mordor, they were not idle. Hôarmûrath of Dír and Ûvatha the Horseman occupied Durthang with several companies of Orcs, and began to regather the forces of Mordor. Over the course of a thousand years they fortified Udûn further than it had been before. The Towers of the Teeth were occupied, and the Black Gate was repaired. Across the gap of Isenmouthe the forts of Carach Angren were constructed. A great ravine was dug and spanned by a single bridge. Behind the ravine a great wall was built and fortified with towers and forts.
The mountain-walls of Udûn became peppered with various constructs. On the inner walls of the mountains between Durthang and the Gates the Man-holds were built to house men of Rhûn. Watchtowers were built on the spur west of Carach Angren, and massive forges, weapon-stores, and armories were built on the east spur. Countless Orc-tunnels were dug in the mountains on either side of the Black Gates and on the inner part of the Ash Mountains.
When Sauron returned to Mordor and Barad-dûr was rebuilt, Orcs returned to Mordor in the thousands. Hundreds of camps, warg-pits, and stores were erected in the Vale of Udûn, which was filled once again with massive armies of Orcs, Wargs, Black Uruks, Easterlings, and Olog-hai. The armies of Mordor prepared for the War of the Ring.
Then the War started. After the ruined city of Osgiliath was taken by the armies of Minas Morgul, another army was sent forth from Udûn. A massive host of Orcs, along with a host of Easterlings, passed through the Black Gate and attacked the island-crossing of Cair Andros. The Orcs conquered the island, and then marched to the Battle of the Pelennor Fields, where they were destroyed. Cair Andros was later reclaimed by men of Gondor.
After the Battle of the Pelennor, the armies of the Free Peoples marched north to assail the Black Gate itself. Most of the armies of Mordor were gathered in Udûn to fight them. When the Gate opened, massive hordes of Orcs and Olog-hai poured out. Orc-tunnels opened up and let loose streams of Orcs into the plain. Men of Rhûn, Khand, and Harad marched out from behind the Ash Mountains. The armies of Gondor and Rohan seemed doomed, even with the arrival of the Eagles of Manwë. But it was then that the Ring was destroyed. Barad-dûr collapsed, and Mount Doom imploded. The tumult from the destruction of Mount Doom and the Dark Tower reached at far as the Vale of Udûn, knocking the Towers of the Teeth over and crushing the Black Gate in the process. Udûn was ruined by the resulting earthquakes and lava flows.
The camps and pits that remained were burned or were buried. The holds in the mountain walls were either buried or empty, and most that survived the destruction were looted afterwards. The servant and creatures of Sauron fled in the the east, and Udûn lay vacant once more.