Ancient Black Demon

Aggressive. As a bonus action, the creature can move up to its speed toward a hostile creature that it can see.

Pounce. If the creature moves at least 20 feet straight toward a creature and then hits it with a claw attack on the same turn, that target must succeed on a DC 18 Strength saving throw or be knocked prone. If the target is prone, the creature can make one bite attack against it as a bonus action.

Regeneration. The creature regains 10 hit points at the start of its turn if it has at least 1 hit point.

Truesight: A creature with truesight can, out to a specific range, see in normal and magical darkness, see invisible creatures and objects, automatically detect visual illusions and succeed on saving throws against them, and perceives the original form of a shapechanger or a creature that is transformed by magic. Additionally, the creature can see into the Ethereal Plane.

Multiattack. The Ancient Black Demon can make 2 bite or claw attacks.

Bite. Melee weapon attack: +8 to hit, reach 5ft., one target. hit: 10(1d10 + 5) piercing damage plus must make a DC 12 Dexterity savings throw, taking 6(2d6) fire damage on a failed save, and have as much on a successful save.

Claw. Melee Weapon attack:+8 to hit, reach 15ft., one target. hit: 10(2d6 + 4) slashing damage.

Spark of Life. Ranged Weapon attack: +4 to hit, reach 60ft., one target. hit: 23(3d10 + 8) fire damage. (can only be used 3 times per day).

Hell Breath. Magic Weapon attack: reach 15ft cone., all enemies inside of the cone must make a DC 14 Dex savings throw. on Failed throw take 18(4d6 + 6) Fire damage and on a successful throw, take no damage. 

opportunity attacks. if a player leaves the attack reach of the Black Demon, the Black Demon can make a bite attack at the enemy.

The Ancient Black Demon has 3 Legendary actions per round

Multiattack. for 1 legendary actions, the Ancient Black Demon can make a multiattack roll

Hell Breath. for 3 legendary actions, the Ancient Black Demon can make a Hell Breath attack