SRD
Death Throes
When the Balrog dies, it explodes and each creature within 30 ft. of it must make a DC 20 Dexterity, taking 20d6 fire damage on a failed save, or half as much damage on a successful one. The explosion ignites flammable objects in that area that aren't being worn or carried, and it destroys the Balrog's weapon.
Fire Aura
At the start of each of the Balrog's turns, each creature within 5 ft. of it takes 3d6 fire damage, and flammable objects in the aura that aren't being worn or carried ignite. A creature that touches the Balrog or hits its with a melee attack while within 5 ft. of it, takes 3d6 fire damage.
Magic Resistance
The Balrog has Advantage on Saving Throws against spells and other magical effects.
Magic Weapons
The Balro'gs weapon attacks are magical.
Multiattack
The Balrog makes two attacks: one with is Longsword and one with its Whip.
Longsword
Melee Weapon Attack: +14 to hit, reach 10 ft., one target. Hit: 3d8+8 slashing damage plus 3d8 lightning damage. If the Balrog scores a Critical Hit, it rolls damage die thrice instead of twice.
Whip
Melee Weapon Attack: +14 to hit, reach 30 ft., one target. Hit: 2d6+8 slashing damage plus 3d6 fire damage and the target must succeed a DC 20 Strength Saving Throw or be pulled up to 25 ft. toward the Balrog.
Teleport
The Balrog magically teleports, along with any equipment it is wearing or carrying, up to 120 ft. to an unoccupied space it can see.