A Balrog is come

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