Arvinador, Matron of Dragons

Chromatic Wrath (Recharges after a Short or Long Rest). If Arvinador would be reduced to 0 hit points, her current hit point total instead resets to 500 hit points, she recharges her Chromatic Flames, and she regains any expended uses of Legendary Resistance. Additionally, Arvinador can now use her "Mythic" reaction and action

Temporal Awarness. Arvinador adds +5 to Initiative.

Legendary Resistance (4/Day). If Arvinadord fails a saving throw, she can choose to succeed instead.

(2/Rest) Action Surge. Arvinadord can take a second action. This feature can only be used once per turn. 

Ignition. When arvinador deals fire damage, the target is ignited by blue fire. They take 1d12 fire damage at the start of their turn. Each time they are ignited this damage increases by 1d12.

Assassinate. If marexia has advantage on a melee attack, that attack deals 31 (9d6) additional piercing damage. 

Scorching Escape. The first time Arvinador is reduced to 0, she is reduced to 1 hit point instead, and the spell Plane Shift is cast on her and she is sent magically to the Elemental Plane of Fire, in one of Arvinador's secret hoards. 

Multiattack. Arvinador may use a breath weapon. Then she may make either a bite or claw attack for each head. Sevirax may replace his bite or claw with casting a spell.

Bite. Melee Weapon Attack: +19 to hit, reach 20 ft., one target. Hit: 23 (2d12 + 10) piercing damage plus 19 (3d12) fire damage. 

Primordial Fire Bite (Mythic). Arvinador's bite damage now deals force damage.

Claw. Melee Weapon Attack: +19 to hit, reach 15 ft., one target. Hit: 21 (2d10 + 10) slashing damage and the target is pushed 15 feet or knocked prone. 

Tail. Melee Weapon Attack: +19 to hit, reach 15 ft., one target. Hit: 23 (2d12 + 10) bludgeoning damage. If the target is a creature, it must succeed on a DC 21 Strength saving throw or be knocked prone.

Breath Weapon [4 Charges, Gain a charge on a 3-6]. Arvinador exhales multicolored flames in a 150-foot cone. Each creature in that area must make a DC 21 Dexterity saving throw. On a failed save, the creature takes 71 (11d12) damage of a type of Arvinador's choosing: Cold, Fire, Poison, or Lightning. On a successful save, the creature takes half as much damage.

(Mythic) Primordial Fire. The fire breath deals force damage instead of fire damage.

Frenzied Lash. Pughulain can make a tail attack with a bonus action. If the target is a Huge or smaller creature, it is grappled (escape DC 20) and is restrained until this grapple ends. Arvinador can have only one creatures grappled this way at a time.

Dash or Disengage. Marexia can allow the body to dash or disengage as a bonus action.

Arvinador has 4 reactions per round. 

(4/Day) Countercast. As a reaction to a spell being cast within 120 feet of Arvinador Sevirax ends that spell if it is 4th level or lower, otherwise Arvinador makes a charisma ability check, with the dc being 10 + the spell's level.

(1/Rest) Cloud Rune. When Arvinador or a creature she can see within 30 feet of her is hit by an attack roll, she can use her reaction to invoke the rune and choose a different creature within 30 feet of her, other than the attacker. The chosen creature becomes the target of the attack, using the same roll. This magic can transfer the attack’s effects regardless of the attack’s range.

(1/Rest) Storm Rune. Arvinador enters a prophetic state for 1 minute or until She's incapacitated. Until the state ends, when she or another creature she can see within 60 feet of her makes an attack roll, a saving throw, or an ability check, she can use her reaction to cause the roll to have advantage or disadvantage. 

Attack. Arvinador makes one Tail attack.

Step Through Chaos(2 Actions). Arvinador teleports 60 feet, everyone adjacent to her when she teleported or adjacent to where she arrives takes 15 (2d12) Acid, Cold, Fire, Lightning, or Poison damage. 

Hurl Through The Elemental Chaos (2 Actions, Mythic Only). Arvinador targets a creature she is grappling. The creature must succeed on a DC 21 Charisma saving throw or take 44 (8d10)force damage and be banished to the elemental chaos. At the start of Arvinador’s next turn, the creature reappears in an unoccupied space within 10 feet of Arvinador.