Valestryx, Embodiment of Flame

Legendary Resistance (5/Day). If Valestryx fails a saving throw, he can choose to succeed instead.

Superheated Scales. Any creature within 100ft of Valestryx experiences sweltering heat and has disadvantage on all concentration and intelligence checks. Creatures who come in physical contact with his scales instantly take 5d10 fire damage, with a DC 20 dexterity saving throw to avoid a further 10d10 fire damage. This effect can only apply once on a creature's turn.

Ultrathermal. Damage dealt by Valestryx bypasses resistance to fire or radiant damage, and treats immunity as resistance. All damage done by Valestryx is considered magical for the purpose of overcoming resistances and immunities.

Oversized Wingtips. When Valestryx is not flying, creatures may choose to target his wingtips rather than his central body, which have 27 AC but does not cause the effects of his Superheated Scales trait, nor his Blazing Sun ability. If his wingtips are hit while he is in flight, Valestryx must make a DC 20 dexterity saving throw or be knocked prone, falling out of the air.
Valestryx's wingtips are positioned 30 feet from his central body on both flanks at all times.

Unerring Fury. Valestryx has advantage on all saving throws in relation to being stunned, paralyzed, petrified, poisoned, prone, restrained and unconscious. When in his Mythic state, he additionally gains +5 bonus to all rolls in the aforementioned categories.

Impossible Self: If Valestryx experiences a change in physical form, he will focus the whole of his will on returning to his original nature. Every minute, he can make a wisdom saving throw contested by the spell save DC of whichever entity may have altered his form. On a success, or if no animate being is responsible, he will return to his original shape and be hostile toward whatever force he identifies to be the cause.

Haltless (2/Long Rest). If Valestryx is at half health or above and is affected by something which would reduce him to zero hit points or otherwise kill, destroy, or erase him, he channels his rage to ignore all damage and persists. If this attack is divine in origin, it does not consume a charge.

Bonfire. Valestryx sheds bright light in a 120-foot radius and dim light for an additional 120 feet. He cannot benefit from being in darkness or heavily obscured areas.

Multiattack. If Valestryx chooses to use Infernal Bite, Searing Talons, or Adamantine Slag, he makes three attacks, of any combination of the three.

Infernal Bite. Melee Weapon Attack: +19 to hit, reach 5ft, one target. Hit: 20 (3d12) piercing damage, plus 50 (6d12 + 15) fire damage.

Searing Talons. Melee Weapon Attack: +19 to hit, reach 25 ft, one target. Hit: 36 (8d8) slashing damage, plus 30 (3d12 + 10) fire damage.

Adamantine Slag. Ranged Weapon Attack: +19 to hit, range 10/100 ft, one target. Hit: 75 (12d8 + 20) fire damage. The target must make a DC 20 dexterity saving throw or become covered in slag, taking 8d10 fire damage at the end of their turn for 1d4 rounds and suffering the restrained condition. A trapped creature may use its action to make a DC 24 strength check to break free.
A creature covered in slag cannot be targeted by this attack, and has +4 to all subsequent saving throws to avoid the slag for ten minutes after it is removed.

 Blazing Current (Recharge 5-6). Valestryx spews a concentrated blast of superheated fire energy in a 60ft cone. Each creature inside must make a DC 24 dexterity saving throw, or take 65 (8d10 + 20) fire damage and 40 (6d12) radiant damage.

Glare (Recharge 5-6). Valestryx focuses his gaze upon a creature within 120 feet of him with whom he shares direct line of sight and who is able to see him. The target must make a DC 23 wisdom saving throw, or be overcome with primordial terror. In addition to applying the frightened condition, they will be compelled to urge their party to escape from the battle as quickly as possible - even considering casting spells or using abilities to 'help others get out safely'. 
The target makes this saving throw again every minute for the next ten minutes, with the DC decreasing to 19 if they are in a quiet, safe environment for three consecutive minutes within that time. If they do not succeed at any point, they are permanently traumatized by Valestryx in a manner only able to be altered by the Wish spell. Integrate this into roleplay at your and your players' discretion. 

Flare Burst (Recharge 3-6). When attacked, Valestryx can unleash a burst of flame, which adds +5 to his armor class during the attack roll, deflects any missile-based attacks, and removes all physical conditions. All creatures within a 30ft radius around Valestryx must make a DC 24 dexterity saving throw or take 80 (15d10) fire damage and be knocked back 20ft and sent prone, otherwise taking half damage and being knocked back 5ft without suffering the prone condition.

Valestryx can take 5 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. Valestryx regains spent legendary actions at the start of his turn.

Nova (Costs 2 Actions). Valestryx flares his wings, producing a massive shockwave. Each creature within 30 feet of him must make a DC 24 dexterity saving throw or take 17 (4d6 + 10) thunder damage and be knocked prone. Valestryx can then fly up to half his flying speed.

Sear (Costs 1 Action). Valestryx channels a gust of superheated air in a line 60 feet long. Every creature within 10 feet of the line must make a DC 24 constitution saving throw or suffer disadvantage on all attack rolls for ten minutes. Effected creatures may repeat the save with a bonus action on their turn, or cure it through cooling themselves down (I:E, throwing a bucket of water on themselves, casting cone of cold, etc...).

Endure (Costs 3 Actions). Valestryx focuses his wrath, knitting his wounds and purging himself of doubt. He regains 100 hit points, and ends one condition afflicting him.

Living Star. Upon being brought to half his hit points, Valestryx's true fire lights, causing him to explode outward with a greater heat than any other. His scales begin to glow a blinding white, liquid metal dripping from between them and from his jaws. When Living Star activates, Valestryx recovers all lost health and releases a burst of fire outward. All creatures within 40 feet are knocked 20 feet away, as well as needing to make a DC 24 dexterity saving throw or take 28 (5d10) fire damage and be blinded until the end of their next turn.

Blazing Sun. When Valestryx is in his Mythic state, simply being in his presence is harmful. Any creature within 20 feet of him must make a DC 24 constitution saving throw at the end of its turn, or suffer increasingly negative effects. These are not cumulative; the most severe overrides the other(s).

1. Creatures who stand under 20 feet from him take 10 (2d10) fire damage.
2. Creatures which are within 10 feet take 25 (5d10) fire damage and suffer disadvantage on all ability checks.
3. Creatures that are within a 5ft radius around Valestryx take 55 (10d10) fire damage and 15 (3d10) thunder damage from the overwhelming blast of heat which he produces, in addition to disadvantage on all rolls. This does not apply to death saving throws.

Flame beyond Fire. The first attack Valestryx makes on his turn which involves fire damage deals an additional 18 (4d8) radiant damage and 18 (4d8) lightning damage, and requires a further DC 20 wisdom saving throw to avoid being blinded until the end of its next turn. If a creature succeeds this saving throw, it no longer must make this save for 24 hours.

Absolute Inferno (3/Week). Using his action, Valestryx begins to charge the ability, producing a visually obvious current of energy around his jaws. On his next turn, he unleashes a 120ft cone of fire energy, as a greater, wider form of his breath attack. Any creature that enters the area for the first time or starts its turn there must make a DC 24 dexterity or constitution saving throw (whichever is higher), or take 10d10 fire damage and 5d10 radiant damage. 
On the turn following his unleashing of this ability, and upon all subsequent turns in which it was maintained, Valestryx may choose to expend his action to "focus", maintaining Absolute Inferno's area and damage, and allowing him to point it in a new direction within 45 degrees of its original orientation. Additionally, after its initial activation, Absolute Inferno gains 8d10 more fire damage and 5d10 radiant damage and the capacity to immediately destroy any creature below a certain threshold (including temporary HP). If a creature is brought below this threshold by the damage dealt by Absolute Inferno, this effect does not apply.

1. After two more rounds of use, the save DC becomes 26. Any creature with less than 25 hit points is instantly and completely destroyed.
2. After three rounds of use, it begins to melt metal, causing damage to any nonmagical metals caught in its blast. It becomes a DC 28 save, and the threshold increases to 50 health or less.
3. After four, Absolute Inferno destroys all nonmagical metals in its area of effect, and begins to damage magical metals. Only a 30 or higher can avoid it, and any creature with less than 100 hp is annihilated.
4. After five it begins to melt magical shields, resistances, and all other forms of arcane defense. A total of 32 or higher is required to avoid the blast, and all creatures caught it which have any less than 200 hit points are summarily destroyed.
5. After six rounds of pure focusing, Absolute Inferno completely destroys all forms of magical shielding to exist. Its save DC becomes 34, and the threshold for unequivocal immolation rises to 400 hit points.
6. For all rounds following, the DC increases by two, and the threshold for immediate annihilation continues to double.

While concentrating on Absolute Inferno, Valestryx is unable to move, and is consistently pushed 10 feet backward relative to the cone's orientation. If Valestryx takes over 125 damage in one round, is stunned, petrified, paralyzed or goes unconscious, his concentration will break and he will be forced to dispel Absolute Inferno.

Any being consumed by Absolute Inferno is completely and perfectly annihilated on the conceptual level; their existence is burnt to ash and the very idea of them is melted away until nothing remains. This cannot be reversed, even through the Wish spell or divine intervention.