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.
While Valinor and the Undying Lands were bathed in the Light of the Trees, all the lands of Middle-earth were plunged into gloom. These were the Ages of Darkness, when Melkor dug the great Pits of Utumno deep beneath the mountains of the North and built many dungeons and domed halls of obsidian, fire and ice. Here were gathered all the evil powers of the World under the Lord of Darkness. Their numbers were in legions and Melkor created many new and dreadful forms. All the serpents of the World were bred in this place, as were the ancestors of the Dragons, the Werewolves, Vampires, Spiders, Fell beasts, and innumberable bloodsucking birds and insects. Cruel spirits, phantoms, wraiths and evil demons stalked the halls of Utumno and haunted the surrounding forests, and were commanded by rebel Maiar: Gothmog the Balrog and Sauron the Sorcerer. Yet with the coming of the Ages of Stars, the power of Utumno was brought to an end.
In the First Age of the Stars, Melkor bred the orcs; twisted lifeforms made from tortured elves who fell in his hands. He also bred Trolls, "made in mockery" of the Ents as stated by Treebeard. Knowing that Melkor would continue to afflict the world, the Valar decided to protect the elves by ending his power in the north by making war against him. Amid the sounding of the trumpets the Valar came out of the west against this new and dreadful evil. There followed the Battle of the Powers in the War for Sake of the Elves in which Utumno was destroyed and Melkor was chained for three long ages.