§ Of the Coming of the Elves and the Captivity of Melkor The Silmarillion

But in the north Melkor built his strength, and he slept not, but watched, and laboured; and the evil things that he had perverted walked abroad, and the dark and slumbering woods were haunted by monsters and shapes of dread. And in Utumno he gathered his demons about him, those spirits who first adhered to him in the days of his splendour, and became most like him in his corruption: their hearts were of fire, but they were cloaked in darkness, and terror went before them; they had whips of flame. Balrogs they were named in Middle-earth in later days. And in that dark time Melkor bred many other monsters of divers shapes and kinds that long troubled the world; and his realm spread now ever southward over Middle-earth.