A Balrog ('baelrQg) is a powerful demonic monster in J. R. R. Tolkien's Middle-earth. One first appeared in print in his high-fantasy novel The Lord of the Rings, where the Fellowship of the Ring encounter a Balrog known as Durin's Bane in the Mines of Moria. Balrogs appear also in Tolkien's The Silmarillion and other posthumously published books. 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. 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. Their power came from their nature as Maiar, angelic beings like the Valar.
Tolkien invented the name "Balrog", providing an in-universe etymology for it as a word in his invented Sindarin language. He may have gained the idea of a fire demon from his philological study of the Old English word Sigelwara, which he studied in detail in the 1930s.
Balrogs appear in the film adaptations of The Lord of the Rings by Ralph Bakshi and Peter Jackson, in the streaming series The Rings of Power, and in computer and video games based on Middle-earth.
According to the fictional history in The Silmarillion, the evil Vala Melkor corrupted lesser Maiar (angelic beings) to his service, as Balrogs, in the days of his splendour before the making of Arda. Upon the awakening 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 the evil giant spider Ungoliant; his scream drew the Balrogs out of hiding to his rescue.
Tolkien's conception of Balrogs changed over time. In all his early writing, they are numerous.