Balor

Huge fiend (demon), chaotic evil

Armor Class 19 (natural armor)
Hit Points 262 (21d12 + 126)
Speed 40 ft., fly 80 ft.

Saving Throws Str +14, Con +12, Wis +9, Cha +12
Damage Resistances cold, lightning; bludgeoning, piercing, and slashing from nonmagical attacks
Damage Immunities fire, poison
Condition Immunities poisoned
Senses truesight 120 ft., passive Perception 13
Languages Abyssal, telepathy 120 ft.
Challenge 19 (22,000 XP)

Death Throes. When the balor dies, it explodes, and each creature within 30 feet of it must make a DC 20 Dexterity saving throw, taking 70 (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 balor's weapons.
Fire Aura. At the start of each of the balor'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 balor or hits it with a melee attack while within 5 feet of it takes 10 (3d6) fire damage.
Magic Resistance. The balor has advantage on saving throws against spells and other magical effects.
Magic Weapons. The balor's weapon attacks are magical.

Actions

Multiattack. The balor makes two attacks: one with its longsword and one with its whip.
Longsword. Melee Weapon Attack: +14 to hit, reach 10 ft., one target. Hit: 21 (3d8 + 8) slashing damage plus 13 (3d8) lightning damage. If the balor 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: 15 (2d6 + 8) slashing damage plus 10 (3d6) fire damage, and the target must succeed on a DC 20 Strength saving throw or be pulled up to 25 feet toward the balor.
Teleport. The balor magically teleports, along with any equipment it is wearing or carrying, up to 120 feet to an unoccupied space it can see.