Balrog of Morgoth; Durin's Bane

Magical Weapon: The Balrog's weapon attacks are magical.

Magical Resistance: The Balrog has advantage on saving throws against spells and other magical effects.

Siege Monster: The Balrog deals twice as much damage to objects and structures.

Fire Aura: At the start of each of the Balrog's turns, each creature within 5 feet of it takes 10 (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 it with a melee attack while within 5 feet of it takes 10 (3d6) fire damage.

Multiattack. The Balrog can make two attacks: one with its Sword of Angband and one with its Whip.

Sword of Angband. Melee Weapon Attack: +15 to hit, reach 10 ft., 1 target. Hit: 16 (2d6+9) fire damage.

Whip. Melee Weapon Attack: +15 to hit, reach 30 ft., 1 target. Hit: 16 (2d6+9) slashing damage plus 10 (3d6) fire damage, and the target must succeed on a DC 20 Strength saving throw or be pulled up to half the distance towards the Balrog (maximum of 15 ft).

Great Leap. The Balrog spreads and meats his mighty wings in a massive leap. It can use this flying leap to move up to 80 ft, landing with a great crash. Any creature within 5 ft of where it lands must make a DC 12 Dexterity check or fall prone.

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

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

Ignite (Costs 1 Actions) (Recharge 5-6). The Balrog bellows and churns its flames, causing its dark fires to roar to greater heights in a great disply. Each creature within a 40 ft range 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, 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 mad add 1d6 to any fire based damage roll, and the range of its Fire Aura is increased to 20 ft. After this ability is used, it cannot be used again until the Balrog has had a long rest.