HD 13 (65) [1+]
Attacks: whip/whip/blade 2d6/2d6/2d10
Saves: F13 +2
Resistances: half damage weapons, requiring magical to damage; full damage from lightning, silvered weapons. Immune to fire damage.
Special defenses: Aura of fear and flame, must make fear save -2 to approach 30', next to heat of balrog causes 1d4 hp fire damage. Weapons shatter after damaging the balrog on a roll of 1-3 on their damage die (magical weapons can be reforged from shards)
Notes from gameplay:
I thought this Balrog would actually be too much for my 5-7 level characters, but they defeated him handily after I fumbled a few attacks and by use of a magic wand.