Vrykos the Crimson Tyrant

Ascendant Fury. When Vyrkos takes damage that would put him below 185 Hp his Fury is triggered by his impending doom. He ascends to a godlike level of power. His hit point total is reset to 600 hit points. He recharges his Breath Weapon, and regains any expended uses of its legendary resistance. 

Legendary resistance (4/day). If Vyrkos fails a saving throw he can choose to succeed instead. 

Empowered Attacks. Vyrkos's attacks count as magical for the purposes of overcoming resistance and immunity to nonmagical attacks.

Fire Absorbtion. When the dragon is subjected to fire damage, it takes no damage and instead regains an amount of hit points equal to the fire damage. 

After Ascendant Fury activates Vyrkos gets the following ability.

Chromatic Aura. Determining Chromatic Aura. Once per round at the top of the round roll a D6. 1:Fire, 2:Ice, 3:Lightning, 4:Poison 5: Acid 6: Roll twice and take both

Vyrkos is sheathed in Chromatic energy. At the start of the Vyrkos's turn, all creatures within 20 feet of it take 21 (6d6) chromatic damage. 

A creature also takes 21 (6d6) Chromatic damage if it touches the dragon or hits it with a melee attack while within 20 feet of it.

Chromatic Absorbtion. While the aura is active Vyrkos becomes immune to the rolled damage type(s) for that round. If he would take damage from the rolled type he would heal for that much instead. 

Multiattack. Vyrkos can make one bite or tail attack and 2 claw attacks.

Bite. Melee Weapon Attack: +18 to hit, reach 25 ft., 1 target. Hit: 21 (2d10 + 10) [Piercing] damage plus 13 (2d12) [force] damage

Claw. Melee Weapon Attack: +18to hit, reach 20 ft., 1 target. Hit: 19 (2d8 + 10) [slashing] damage. 

Tail. Melee Weapon Attack: +18to hit, reach 20 ft., 1 target. Hit: 19 (2d8 + 10) [bludgeoning] damage. The target must succeed on a DC26 strength save or be knocked prone. 

Breath Weapon (Recharge 5-6) Vyrkos exhales a blast of energy in a 300ft cone. Each creature in that area must make a DC 27 dexterity saving throw. On a failed Save the creature takes 120 (20d12) Fire damage. On a successful save the creature takes half as much. 

After Ascendant Fury activates 

Breath Weapon (Recharge 5-6) Vyrkos exhales a blast of energy in a 300ft cone. Each creature in that area must make a DC 27 dexterity saving throw. On a failed Save the creature takes 120 (20d12) Of the rolled chromatic damage type (determined by Chromatic aura). On a successful save the creature takes half as much. If two chromatic types are active the damage is 60 for each type. 

Cunning Tactician: When a creature makes an attack roll against Vrykos, he can impose disadvantage on that roll. He can use this ability once per round, refreshing at the end of his next turn.

Vyrkos 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. Vrykos regains spent legendary actions at the start of its turn.

Attack. Vrykos makes Claw or a tail attack.

Wing Attack (Costs 2 Actions). Vyrkos beats its wings. Each creature within 30 feet of the dragon must succeed on a DC 26 Dexterity saving throw or take 17 (2d6 + 10) bludgeoning damage and be knocked prone. Vyrkos can then fly up to half his flying speed.

Arcane Spear (cost 3 actions). Vyrkos creates four spears of magical force. Each spear hits a creature of Vyrkos choice that he can see within 120 feet, dealing 12 (1d8+8) force damage to its target, then disappears. 

Fire Surge (Costs 2 Actions): Vyrkos unleashes a wave of fire in a 15-foot radius centered on himself. Each creature in that area must make a DC 27 Dexterity saving throw, taking 21 (6d6) fire damage on a failed save or half as much on a success.

Intimidating Presence (Costs 1 Action): Vyrkos can attempt to frighten one creature he can see within 60 feet. The target must succeed on a DC 27 Wisdom saving throw or be frightened until the end of its next turn.

Winged Maneuver (Costs 2 Actions): Vyrkos can reposition himself. He can move up to his flying speed without provoking opportunity attacks, allowing him to escape danger or reposition for a better attack.

If ascendant fury is active Vrykos can also choose to use the following as legendary actions. 

Bite. Vyrkos makes one bite attack.

Chromatic Flare (Costs 2 Actions): Vyrkos flares with chromatic energy. Each creature in a 60-foot radius sphere centered on Vrykos must succeed on a DC 26 Dexterity saving throw or take 22 (5d8) damage of the rolled type, determined by the Chromatic Aura. On a successful save, creatures take half as much damage.