The ranks of the orcs had opened, and they crowded away, as if they themselves were afraid. Something was coming up behind them. What it was could not be seen: it was like a great shadow, in the middle of which was a dark form, of man-shape maybe, yet greater; and a power and terror seemed to be in it and to go before it.
It came to the edge of the fire and the light faded as if a cloud had bent over it. Then with a rush it leaped across the fissure. The flames roared up to greet it, and wreathed about it; and a black smoke swirled in the air. Its streaming mane kindled, and blazed behind it. In its right hand was a blade like a stabbing tongue of fire; in its left it held a whip of many thongs.
'Ai! ai! ' wailed Legolas. 'A Balrog! A Balrog is come! '
Gimli stared with wide eyes. `Durin's Bane! ' he cried, and letting his axe fall he covered his face.
'A Balrog,' muttered Gandalf. `Now I understand.' He faltered and leaned heavily on his staff. `What an evil fortune! And I am already weary.'
The Bridge of Khazad-dûm, The Fellowship of the Ring, the Lord of The Rings, by J.R.R. Tolkien