A Balrog is a fictional demonic creature from J. R. R. Tolkien's Middle-earth fantasy writings and appears as an enemy of Gandalf in The Lord of the Rings. Such creatures first appeared in print in his novel The Lord of the Rings, though they figured in earlier writings that posthumously appeared in The Silmarillion and other books.
Balrogs are described as tall, menacing humanoid beings, with the ability to shroud themselves in fire, darkness, and shadow. They frequently appeared armed with fiery whip "of many thongs", and occasionally used long swords. In Tolkien's later conception, they could not be casually destroyed; significant power was required. Only dragons rivalled their capacity for ferocity and destruction, and during the First Age of Middle-earth, they were among the most feared of Morgoth's forces.
The Balrogs were Maiar, of the same order as Sauron, Saruman and Gandalf. They were seduced by Melkor, who corrupted them to his service in the days of his splendour before the making of Arda.
At the dawn of the First Age, upon the waking of the Elves, the Valar captured Melkor and destroyed his fortresses Utumno and Angband. But they overlooked the deepest pits, where, with many of Melkor's other allies, the Balrogs fled into hiding. When Melkor returned to Middle-earth from Valinor, now bearing the epithet Morgoth, he was attacked by Ungoliant, a spider-like creature; and his piercing scream drew the Balrogs out of hiding to his rescue.
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Official Site: JRR Tolkien Estate
Links: wikipedia Encyclopedia of Arda The Truth about Balrogs