Balrog

Fire Aura. At the start of each of the Balrog's turns, each creature within 10 feet of it takes 7 (2d6) fire damage and 3 (1d6) necrotic damage, and flammable objects or beings in the aura that aren't being worn or carried ignite. While on fire in this way, the object or being takes 5 (1d10) fire damage. A creature that touches the Balrog or hits it with a melee attack while within 5 feet of it takes 7 (2d6) fire damage and 3 (1d6) necrotic damage, and is set on fire unless already on fire. The balrog also emits dim light for 60ft around them. 

Charge. If the balrog moves at least 10 feet straight toward a target and then hits it with a gore attack on the same turn, the target takes an extra 9 (2d8) piercing damage. If the target is a creature, it must succeed on a DC 21 Strength saving throw or be pushed up to 10 feet away and knocked prone.

Water Susceptibility. If the balrog is fully submerged in water, it's fires become quenched and it loses its fire aura trait as well as the additional fire damage dealt by its attacks until it can dry itself completely. 

Multiattack. The Balrog makes 2 longsword attacks.

Flaming Longsword. Melee Weapon Attack: +12 to hit, reach 10 ft., 1 target. Hit: 25 (4d8 + 7) slashing damage, and 5 (1d10) fire damage.

Gore. Melee Weapon Attack: +12 to hit, reach 5 ft., one target. Hit: 24 (5d6 + 7) bludgeoning damage, 5 (1d10) fire damage and 5 (1d10) necrotic damage. 

Fire Whip. Melee Weapon Attack: +6 to hit, reach 30 ft., 1 target. Hit: 10 (2d8 + 1) slashing damage, and 5 (1d10) fire damage. The target must make a strength saving or be pulled into the balrog's aura and taking the damage. 

Demonic Fear(1/Day). Everyone within a 60 foot radius must succeed on a DC 17 Wisdom saving throw or be frightened of the Balrog. A creature frightened in this way may make another saving throw at the start of their turn.