The Balrog
Balrogs, also known as the Valaraukar, were Maiar that were seduced and corrupted by Melkor into his service.
Third Age
In TA 1980, a Balrog awoke in Moria when the Dwarves had mined too deep for Mithril. It drove the Dwarves out of their home and slew King Durin VI, and the Balrog was thereafter called "Durin's Bane".
During the War of the Ring, the Fellowship of the Ring passed through Moria and encountered Durin's Bane, which pursued them to the Bridge of Khazad-dûm. Gandalf the Grey fought the Balrog, allowing the Fellowship to escape Moria. Both fell into the abyss, but the battle continued at the peak of Zirakzigil. Finally, it ended, but both Gandalf and Durin's Bane were slain in the process. Gandalf was later "sent back" by the Valar, as Gandalf the White.
A single Balrog, who became known as Durin's Bane, alone managed to drive the Dwarves of Moria from their ancient and supremely fortified nation-state, which was at the time the greatest kingdom of Dwarves that had ever been. It also contended with Gandalf, and shattered the side of a mountain with physical might alone. The Balrogs were considerably bodily agile, such that their passing is once described as a “tempest of fire”.
In other writings, Balrog is derived from ñgwalaraukô ("demon").
http://lotr.fandom.com/wiki/Balrogs
"Return of the king" was definitely the best of trilogy. Epic battles and epic twists.
