Bone Dragon (Actheros form).

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

Aura of death. The Bone Dragon excretes a continuous Aura of death in a 30 ft circle. Each creature that enters or starts its turn within the Aura must succeed on a DC 26 Constitution saving throw or take 8d6 necrotic damage or half as much on a success. 

Dark soul. While in an area of dim light or darkness, the Bone Dragon can use a bonus action to turn invisible. He can end this at any time he chooses. If the Bone Dragon makes an attack of takes an action he ends the invisibility. 

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

Multiattack. The dragon can use its Frightful Presence. It then makes four attacks: one with its bite and two with its claws.

Bite. Melee Weapon Attack: +18 to hit, reach 15 ft., one target. Hit: 21 (3d10 +9) piercing damage plus 14 (5d6) necrotic damage.

Claw. Melee Weapon Attack: +18 to hit, reach 10 ft., one target. Hit: 17 (3d6 + 9) slashing damage.

Tail. Melee Weapon Attack: +18 to hit, reach 20 ft., one target. Hit: 19 (3d8 +9) bludgeoning damage.

Teleport. The Bone Dragon magically teleports, along with any equipment it was wearing or carried, up to 100 ft to an unoccupied space it can see.

Frightful Presence. Each creature of the dragon's choice that is within 120 feet of the dragon and aware of it must succeed on a DC 26 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 dragon's Frightful Presence for the next 24 hours.

Change Shape. (1/day). The Bone Dragon is able to polymorph into Actheros. 

Death Breath (Recharge 5–6). The dragon exhales fire in a 90-foot cone. Each creature in that area must make a DC 24 Dexterity saving throw, taking 91 (26d6) Necrotic damage on a failed save, or half as much damage on a successful one.

The dragon can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature's turn. The dragon regains spent legendary actions at the start of its turn.

Attack. Bone Dragon makes an attack. 

Move. The Bone Dragon moves up to half its speed.

Wing Attack (Costs 2 Actions). The Bone Dragon beats its wings. Each creature within 15 feet of the dragon must succeed on a DC 26 Dexterity saving throw or take 17 (2d6 + 10) bludgeoning damage and be knocked prone. The dragon can then fly up to half its flying speed.