War of Wrath
Demonic creatures of fire and shadow, Balrogs were fallen Maiar, loyal to the first Dark Lord, Morgoth. They participated in the wars of the First Age of Middle-earth but were mostly destroyed during the War of Wrath which ended the Age. By the Third Age, the only remaining Balrog was "Durin's Bane," the Balrog of Moria, killed by Gandalf.