Utumno (also known as Udûn) is the first fortress of Melkor in the far north of Middle-earth.
It 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 Ered Engrin, where the light of Iluin was very dim.
Utumno was his base of operations for 1149 Valian Years, and from here he assaulted the Two Lamps, and began corrupting Arda. It was also here that the first captured Elves were dragged to, and breeding of the Orcs began. It was the largest architectural structure ever to have existed, surpassing Angband and the Thangorodrim (35,000 ft. according to a sketch by Tolkien) and the later Barad-dûr.
Udûn is also the name of a valley in northern Mordor. The word means "Hell".