Balrogs will be spawned starting from a depth of -1800.
They are extremely powerful (HP: 200-600; Damage: 20), thus will be very rare.
If you manage to defeat one, it may drop a fire whip.
You can tune this mod through Settings/Advanced Settings/Mods/mobs_balrog
This mob was originally developed for the Lord of The Rings Game, I've just made it a separate mod.