Balrogs

Valaraukar in Quenya, demons of power: spirits of the order of the Maiar whom Melkor drew to himself in the ages before the Sun and bent to his service. They stood roughly in the shape of Men but at twice the height, wrapped in black smoke and flame, their manes kindling as they ran, their eyes yellow as furnace mouths and their long arms tipped with steel. A whip of many flaming thongs was the chief weapon of a Balrog, and they went helmed and mailed in iron. Tolkien's late writings set their number very low, no more than three and at most seven. So heavy was the terror they carried that Morgoth's own servants gave way before them.