Bone Feaster

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

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

Regeneration. The Bone Feaster regains 25 hit points at the start of his turn as long as he is not unconscious. 

Magic Weapons. Bone Feaster's weapon attacks are magical.

Multiattack. The Bone Feaster can make two Blade of Cursed Bones, one bite, and one tail attack, or he can use a blade of cursed bones attack and a breath attack.

Blade of Cursed Bones. Melee Weapon Attack: +18 to hit, reach 10 ft., one target. Hit: 42 (7d8 + 8) slashing damage plus 26 lightning damage. If the bone feaster scores a critical hit, it rolls damage dice three times, instead of twice.

Bite. Weapon Attack: + 19 to hit, reach 15 ft., one target. Hit: 24 (4d6 + 10) slashing damage.

Frightful Presence. Each creature of the Bone Feaster's choice that is within 120 feet of the dragon and aware of it must succeed on a DC 22 Wisdom saving throw or become frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. If a creature's saving throw is successful or the effect ends for it, the creature is immune to the Bone Feaster's Frightful Presence for the next 24 hours.

Hell Acid Breath (Recharge 4–6). The Bone Feaster exhales acid and fire in a 90-foot line that is 10 feet wide. Each creature in that line must make a DC 24 Dexterity saving throw, taking 66 acid damage and 66 fire damage on a failed save, or half as much damage on a successful one.

The Bone Feaster can take 4 legendary actions, choosing from the options below. Only one legendary action can be used at a time and only at the end of another creatures turn. 

Evil Calling (Costs 2 Actions). The Bone Feaster summons 5 Carnage Fiends, which appears in an unoccupied space within 60ft of him, acting as his ally's. 

Hell Acid Breath (Costs 2 Actions). The Bone Feaster exhales acid and fire in a 90-foot line that is 10 feet wide. Each creature in that line must make a DC 24 Dexterity saving throw, taking 66 acid damage and 66 fire damage on a failed save, or half as much damage on a successful one.

Move (Costs 1 Action). The bone feaster moves up to his movement speed. This does not provoke opportunity attacks. 

Rageful Smoke (Costs 3 Actions). The bone feaster expels a toxic smoke in a 60-foot radius. The cloud spreads around corners, and that area is heavily obscured to creatures other than the bone feaster. Each creature other than the bone feaster that ends its turn there must succeed on a DC 25 Constitution saving throw, taking 26 (5d10) poison damage on a failed save, or half as much damage on a successful one. A strong current disperses the cloud, which otherwise disappears at the end of the bone feaster's next turn.