Utumno was built by Melkor after his first expulsion from Arda. The Valar had by this time created the Two Lamps, and Utumno was built around Valian Year 3400 under the Iron Mountains, where the light of Illuin was very dim.
Melkor used Utumno as his base of operations for 1,149 Valian Years (11,000 solar years), and from here he destroyed the Two Lamps and began his corruption of Arda. Utumno was also where the first captured Elves were taken and the breeding of Orcs began. It was the largest architectural structure ever to have existed, surpassing Angband and Thangorodrim in the First Age and Barad-dûr in the Second and Third Age.
Utumno was laid waste in the Years of the Trees, Valian Year 1099, in the war that the Valar began against Melkor because of their discovery of the Elves. Melkor was chained and dragged as a captive to Valinor.
Melkor established a second fortress at the western end of the Ered Engrin to act as the first line of defense for Utumno. This became Angband, which was at first held by Sauron. After the destruction of Utumno, Melkor chose to rebuild and fortify Angband as his lair.
Udûn is also the name of the circular valley in northern Mordor.