Utumno (in Quenya) or Udûn (in Sindarin) was a fortress of Melkor in the far north of Middle-earth. It was the first and greatest of Melkor’s citadels, delved in the earliest days. It was the home to hosts of demons, the fallen Ainur who allied with Melkor, and numerous monsters of corruption were made in mockery of Middle-earth’s naturally intended creatures and beings.
Utumno was built by Melkor after his first expulsion from Arda. The Valar had by this time created the Two Lamps. Utumno was built under the Iron Mountains, where the light of Illuin and Ormal was dim and cold. Though the Valar did not yet know it, from this place, “the blight of his hatred flowed out thence, and the Spring of Arda was marred.”
Melkor used Utumno as his base of operations from the Valian Year 3400 until Y.T. 1090, when the Valar assailed Melkor’s fortress, which fell in Y.T. 1099. From there he had destroyed the Two Lamps, so that the Powers left Almaren, their dwelling-place in Middle-earth, and removed into the West. He then began his corruption of Arda. Utumno was also where the first captured Elves were taken and the creation of Orcs began as a mockery of the Firstborn.
Utumno was laid waste in Y.T. 1099, in the war that the Valar began against Melkor for the sake of the Elves. The Valar attacked Udûn in full force, and destroyed it utterly, carrying its master back to Valinor as their prisoner. Melkor was chained and dragged as a captive to Valinor.
Melkor had established a second and lesser fortress at the western end of the Ered Engrin to act as the first line of defence for Utumno from Aman. 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.