What fiery creature did Gandalf fight in Moria?

The fiery creature Gandalf fought in Moria was a Balrog. The creature appeared when the Fellowship travelled through the abandoned Dwarven city of Moria, also known as Khazad-dûm, and disturbed the Orcs in the Chamber of Mazarbul.

The Balrog was one of the ancient spirits corrupted by Morgoth, Tolkien’s first Dark Lord. It was associated with fire, shadow, a flaming sword and a many-thonged whip. Although the creature is often imagined as a generic demon, Tolkien presents it as an ancient and immensely powerful being from the earliest ages of Middle-earth.

Gandalf confronted it on the Bridge of Khazad-dûm, breaking the bridge beneath it before being dragged into the abyss. Their battle continued underground and eventually ended on the peak of Zirakzigil, where Gandalf defeated the Balrog after a fight lasting two days and nights. Gandalf died from his wounds but returned as Gandalf the White.

A common mix-up is confusing the Balrog with the cave troll, which attacked the Fellowship earlier in Moria.