Balrog

Fire Aura. At the start of each of the balrog's turns, each creature within 5 feet must make a constitution save (DC 13) or take 1d6 fire damage, and flammable objects in the aura that aren't being worn or carried ignite. A creature that touches the balrog or hits it with a melee attack while within 5 feet of it must also make the constitution save or take 1d6 fire damage. The balrog also sheds bright light in a 30-foot radius and dim light for an additional 30 feet.

Legendary Resistance (3/Day). If the Balrog fails a saving throw, it can choose to succeed instead.

Magic Resistance. The balrog has advantage on saving throws against spells and other magical effects.

Magic Weapons. The balrog's weapon attacks are magical.

Multiattack. The balrog can attack twice during its turn.

Longsword. Melee Weapon Attack: +14 to hit, reach 10 ft., one target. Hit: 2d8 + 8 slashing damage plus 3d8 fire damage. If the balrog scores a critical hit, it rolls damage dice three times, instead of twice. 

Whip. Melee Weapon Attack: +14 to hit, reach 30 ft., one target. Hit: 1d6 + 4 slashing damage plus 2d6 fire damage, and the target must succeed on a DC 20 Strength saving throw or be pulled up to 25 feet toward the balrog.

The Balrog can take 1 legendary action, choosing from the options below. The Balrog regains spent legendary actions a the start of its turn.

Roar of the Valaraukar (Costs 1 Action) (Recharge 5-6). The Balrog lets loose a shrieking roar. Every creature in a 20 ft. cone must make a DC 15 Dexterity saving throw or be thrown 10 ft. from the balrog, taking 2d8 fire damage and falling prone; they take half the fire damage on a success. 

Ignite (Costs 1 Action) (1/day). The Balrog bellows and churns its flames, causing its dark fires to roar to greater heights. Each creature within 40 ft. of the Balrog that can see it must make a DC 18 Wisdom Save or become frightened for 1 minute. They may make another save at the start of their turn, and on a success or if they resisted the original roll, they are immune from this ability for 24 hours. The Balrog stays in this ignited state 1d4 rounds. It may add 1d6 to its damage roll when it deals fire damage, and the range of its Fire Aura is increased to 20 ft. This ability can be used once per day.