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Balrogs, also called Valaraukar, were originally Ainur created by Ilúvatar, probably those who joined Melkor during his discordance in the Music of the Ainur. After entering into Eä, they were Maiar, lesser spirits at the service of the Valar.

In Tolkien's Elvish language Sindarin, Balrog means “Demon of Might”. They began as servants to Sauron's predecessor Morgoth. After Sauron, Balrogs were the most powerful and most terrifying of his servants. They are fire spirits, but also shrouded in darkness. Most of the Balrog died in the Great Battle, but a few survived by hiding in the depths.

Balrogs are tall and menacing beings who can shroud themselves in fire, darkness, and shadow. They are armed with fiery whips "of many thongs", and occasionally use long swords. In Tolkien's later conception, Balrogs could not be readily vanquished—a certain stature was required by the would-be hero.