Maiar, servants of Melkor, spirits of fire.
Also Demon of Might. Quenya name is Valaraukar.
The spirits who first adhered to Melkor in the days of his splendour, and those that became most like Melkor in his corruption. Their hearts were of fire, but they where cloaked in darkness. They carried with them whips of flame. Of all Melkor's servants they are the most powerful after Sauron. Their lord, Gothmog, held only slightly less power and authority than Sauron himself. In Tolkiens early works there were scores of Balrogs but as he increased them greatly in power he later changed this number to "no more than seven".
Most of the Balrogs were destroyed in the War of Wrath, save some that fled and hid in caverns inaccessible at the roots of the earth. One of them hid in the MistyMountains until the Dwarves woke him up.
Only three persons are known to have "defeated" a Balrog and "defeated" is a term that is used loosely since all three died in the process:
Ecthelion of the Fountain fought Gothmog, Lord of Balrogs and High Captain of Angband during the Fall of Gondolin.
Glorfindel fought a Balrog in Cirith Thoronath, on the same occasion, protecting the elves fleeing The Fall of Gondolin.
The third person to fight "succesfully" with a Balrog was Olórin in the Mines of Moria. He was later sent back, resurrected from the dead, with greater power as Gandalf the White.
All three of them paid for the effort with their lives.
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