Utumno (Q. ‘the Pit’; S. Udûn ‘Hell’), was the name given to the first abode of Melkor in Middle-earth, which he made back in the Elder Days. Delved deep into the flesh of the Earth, Utumno consisted of many hundreds of caves, tunnels, corners, and rooms, allowing things to remain hidden for a very long time.
It was from here that he assaulted the Two Lamps and began his corruption of Arda, and also where the first captured Elves were taken for the breeding of Orcs. He gathered many other evil beings in his fortress – werewolves, vampires, giant spiders, fell beasts, Trolls (which he bred in mockery of the Ents), cruel spirits, wraiths, and evil demons.
Utumno became deserted in the Years of the Trees, after Melkor was defeated by the Valar in the Battle of the Powers and held captive in Valinor; while parts of his fortress had escaped destruction, he never returned to it.