The balrog, &, is the most powerful of the ordinary major demons in NetHack. Like in its original concept, it is armed with bullwhip and broadsword. Unlike other major demons, they are incapable of summoning additional demons.
The balrog can steal your weapon with its bullwhip.
Encyclopedia entry Edit
... 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!'