The Balrog encountered in Moria in J.R.R. Tolkien’s The Lord of the Rings is called Durin’s Bane.
The name means that this Balrog became the doom of Durin’s folk. It awoke beneath Khazad-dûm after the Dwarves mined too deeply for mithril. The creature killed Durin VI, and his people eventually abandoned the underground city, which became known as Moria.
The Fellowship encounters Durin’s Bane while crossing the abandoned mines. Gandalf confronts it on the Bridge of Khazad-dûm, where both fall into the depths. Their battle continues through subterranean waters and reaches the peak of Zirakzigil before Gandalf defeats the Balrog, although he dies from his wounds and later returns as Gandalf the White.
Durin’s Bane is not the same character as Gothmog, a Balrog lord named in Tolkien’s First Age stories. Nor is it the Watcher in the Water, a separate creature outside the Doors of Durin. These distinctions matter because Moria contains several different threats.