In The Lord of the Rings: The Fellowship of the Ring, Gandalf fights a Balrog on the Bridge of Khazad-dûm.
The Balrog appears after the Fellowship travels through the mines of Moria. Gandalf identifies the creature as a being of great power and confronts it on the narrow bridge. His command, “You shall not pass,” gives the group time to escape, but the Balrog’s whip catches him and pulls him from the bridge.
Both Gandalf and the Balrog fall into the depths beneath Moria. Their struggle continues far below, ending with Gandalf defeating the creature atop the peak of Zirakzigil. The battle also leads to Gandalf’s transformation from Gandalf the Grey into Gandalf the White.
A Balrog is not simply a large Orc or an ordinary demon. In Tolkien’s mythology, Balrogs are corrupted spirits of fire known as Maiar. The Moria creature is commonly called the Balrog of Moria, and the film presents it as a towering, shadowy, flame-wreathed figure.