The Balrog

The Balrog was an ancient fiery demon from ancient times who had fled to the depths of earth. Woken by the greedy Dwarves in their search for mithril, it had taken the halls of Khazad-dum as its foul domain.

Bridge of Khazad-dum
After the Fellowship of the Ring escapes Balin's Tomb, the Orcs pursue them to the Bridge of Khazad-dum, where they break lines for the Balrog to pass. The fiery demon of Shadow ignites his sword of flams and his whip before confronting the wizard Gandalf the Grey. The two fall with the bridge into the chasm bellow before an ultimate fight on the top of the mountain where the Balrog is destroyed.

Trivia

 * The Balrog in Ralph Bakshi's The Lord of the Rings has the head of a lion and the feet of a horse-like creature. It has bat-like wings which it uses to hover for a time before descending back to ground.
 * The Orcs seem to be allied with the Balrog as they allow it to pass and do not seem cowed by its presence like in the novel.