Gandalf confronts a Balrog on the Bridge of Khazad-dûm in J.R.R. Tolkien’s The Fellowship of the Ring.
The creature is identified as a Balrog, a powerful being of fire and shadow. In the novel, the Fellowship travels through the Mines of Moria and is pursued by the Balrog after the death of the dwarf Balin’s company. Gandalf stands on the Bridge of Khazad-dûm and breaks the bridge beneath the creature, but he falls with it into the depths.
The Balrog is later named Durin’s Bane because it killed Durin IV, a king of Khazad-dûm, long before the Fellowship arrived. Tolkien’s legendarium describes Balrogs as corrupted Maiar who served Morgoth in the First Age.
The scene is often remembered from the film adaptation, whose famous line is “You shall not pass!” The book’s event is the same in broad outline, but the creature’s wider history comes from Tolkien’s other writings.