Barioth(?)

Ice Walk.  Barioth can move across and climb icy surfaces without needing to make an ability check. Additionally, difficult terrain composed of ice or snow doesn't cost it extra movement.

IceBlight. Ice Pillar and Artic Wind will inflict the enemy with Iceblight on a failed save.  Creatures inflicted with Iceblight will take (1d4) cold damage and can not use Reactions.  It can not be cured until an appropriate heat source is provided or after an hour has past.

Adamantium Bone. When attacking with Bite or Claw, your Critical range is 18 - 20 and Barioth can not be Critically hit.

Enraged. When Barioth's health is 1/4(55), this passive will activate. Barioth is granted one extra Action and one extra Reaction. Can not use Multiattack with extra action.

Multiattack. Barioth can make two attacks. The same attack can not be used twice, Ice Pillar  and Artic Wind  can not be used with this feature.

Bite. Melee Attack: +10 to hit, reach 5 ft., 1 target. Hit: 23 (2d10 + 7) Piercing + 8 (1d10) Cold damage. 

Claw. Melee Attack: +10 to hit, reach 5 ft., 1 target. Hit: 16 (2d8 + 7) Slashing + 3 (1d6) Cold damage.

Howl. Melee Attack: Area of Effect: Self (30 ft. radius, sphere). DC 19 constitution save. Hit: 7 (1d10) Force damage. On failed save, enemy is stunned and deafened. On success save, enemy takes half damage and is not stunned but still deafened.

Tail Sweep. Melee Attack: Area of Effect: Self (10 ft. radius, half circle) DC 17 dexterity save. Hit: 15 (2d6 + 7) Bludgeoning damage. On failed save, enemy is knocked prone. On success save, enemy takes half damage and is not knocked prone.

Dragon Howl. Ranged Attack: Area of Effect: 10 ft. wide, 120 ft. line, self. DC 15 constitution save. Hit: # (3d8) Cold damage. On failed save, enemy is frozen (petrified) and inflicted with IceBlight. On a success save, enemy takes half damage and no condition is inflicted. 

• On impact with a large object, Dragon Howl will explode at point of impact and cause the same damage and effect again in a 25 ft sphere. 
• Most effective when flying due to trajectory, causing damage to an area with the line and impact.

Artic Wind. Ranged Attack: Area of Effect:  15 ft. radius, 50 ft. high, range 30 ft. DC 15 dexterity save. Hit: 15 (3d6) Cold damage. On failed save, enemy is knocked up 40 ft. and knocked back 20 ft., and inflicted with IceBlight. On success save, enemy is knocked back 15ft. and no other ailment is inflicted.

• Lasts for 2 rounds(12 seconds). any creature that enters will take the same damage and effect.
• Fall damage.
• Physical projectiles can not pass through as they are blow away in some random direction.

Barioth can spam aoe moves but that would be no fun.

Triple Back Flip Quadra Spin. When successfully saving against a spell, take no damage instead and moves 40 ft in a line without provoking opportunity attacks.  Can land on any solid surface or stop in the air flying.  "It literally does a triple back flip out of a fireball, yes a dragon does that now."

Wing Guard. You can impose a disadvantage on any attacks made at you and a advantage on saving throws against damaging attacks  "And now it has adamantium bone shield?!  Is the CR correct? What the hell am I fighting?"                     

Reactions are Barioth's main method to get air time.

Barioth 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.

Move Quick (Costs 1 Action). Increase AC by 10(26). Move your maximum movement.  Can be Opportunity Attacked

Quick Attack (Costs 2 Actions). Increase AC by 4(20). Move your maximum movement and do a single Claw attack against a single creature you pass.  Can be Opportunity Attacked

Snow Shroud (Costs 3 Actions). Melee Attack: Area of Effect: 5 ft. self, Hit: (3d10) Slashing damage. Knockback any creature hit 10 ft. Barioth rotates repeatedly at an incredible speed on the ground. causing a snow cloud in a 50 ft. sphere. It lasts 4 rounds(24 seconds) and the area is heavily obscured. There is no save.