In J.R.R. Tolkien’s The Lord of the Rings, Gandalf defeats the Balrog after battling it on the Bridge of Khazad-dûm and later on the peak of Zirakzigil. The fight follows the creature’s attack on the Fellowship in Moria.
Gandalf confronts the Balrog so the others can escape. He breaks the Bridge of Khazad-dûm beneath the demon, but the Balrog’s whip catches him and pulls him into the abyss. The two continue fighting through the underground depths and eventually reach the mountain summit. Gandalf kills the Balrog, but he also dies from the struggle.
He later returns as Gandalf the White, having completed a transformation that gives him greater authority. The Balrog is identified as a creature of shadow and flame, and Tolkien’s later writings establish it as a corrupted Maia, a spiritual being of the same broad order as Gandalf and Sauron. The name “Balrog” comes from Tolkien’s invented languages and is associated with power and terror. The creature in Moria is also called Durin’s Bane.