In the northeast of Middle-earth during the Ages of the Lamps, Melkor built a mighty fortress ringed with mountains, and he called it Utumno. There he plotted against the other Valar and gathered to himself monsters, like Balrogs, Werewolves and Great Spiders.

After Melkor destroyed the Lamps of the Valar, Utumno's empire on Middle-earth expanded through the Ages of Darkness that followed.

The Valar at last made war on Utumno at the end of the First Age of the Stars. This war was the War of the Powers, and at the end of that war Utumno was entirely destroyed and Melkor captured and put in chains.
Encyclopedia entry originally written by Gil_Estel101