Name: balrog
Difficulty: 20
Base level: 16
Base experience: 559
Speed: 5
Base AC: -2
Base MR: 75
Alignment: -14
Frequency: Very rare, only in Gehennom
Genocidable: No
ATTACKS:
    Projectile as weapon, or if in range, uses melee weapon for 8d4
    Projectile as weapon, or if in range, uses melee weapon for 4d6
Weight: 1450
Nutritional value: 400 (but leaves no corpse)
Size: large
Resistances: fire, poison
Resistances conveyed by eating: None

A balrog can fly/float. It can see invisible creatures. It is poisonous if eaten.

... 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