In J.R.R. Tolkien’s The Lord of the Rings, Gandalf leads the confrontation with the Balrog in Moria.
The Fellowship encounters the Balrog beneath the Mines of Moria after disturbing the ancient underground realm. Gandalf stands on the Bridge of Khazad-dûm and prevents the creature from crossing, declaring that it cannot pass. The Balrog breaks the bridge, but its fiery whip catches Gandalf and pulls him into the abyss.
Gandalf survives the fall and continues fighting the Balrog through the depths of Moria and up to the peak of Zirakzigil. He defeats it, but dies from his wounds. He later returns as Gandalf the White, taking a greater role in the struggle against Sauron.
The Balrog is a Maia, a powerful spiritual being corrupted by Morgoth. It is sometimes called Durin’s Bane, but that name identifies the specific Balrog beneath Moria, not the wizard or the Dwarven king Durin.