A Balrog 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 his legendarium. 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.

Middle-earth is the setting of much of the English writer J. R. R. Tolkien's fantasy. The term is equivalent to the Miðgarðr of Norse mythology and Middangeard in Old English works, including Beowulf. Middle-earth is the human-inhabited world, that is, the central continent of the Earth, in Tolkien's imagined mythological past. Tolkien's most widely read works, The Hobbit and The Lord of the Rings, are set entirely in Middle-earth. "Middle-earth" has also become a short-hand term for Tolkien's legendarium, his large body of fantasy writings, and for the entirety of his fictional world.