... 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
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!'
[ The Fellowship of the Ring, by J.R.R. Tolkien ]