Ashardalon

Chromatic Awakening. (Recharges after a Short or Long Rest).                                              If Ashardalon would be reduced to 0 hit points, his current hit point total instead resets to 500 hit points, he recharges his breath weapon, and regains any expended uses of legendary resistance. Additionally, Ashardalon can now use the options in the "Mythic Actions" section for 1 hour. Award the party an amount of XP that you think would be a good amount.

Legendary Resistance (4/Day). If Ashardalon fails a saving throw, he can chose to succeed instead.                                 

 Ashardalon's Scorching Presence. Any creature that hits Ashardalon with a melee attack or starts it's turn within 30 feet of Ashardalon has to make a DC 25 dexterity saving throw or take 4d10 fire damage on a fail and half as much on a success, and any non magical weapons and armor or any thing that is not being worn or carried are heated up and lit on fire. Any creature that is touching these things take 2d10 fire damage and if it is a weapon they have disadvantage on all attack roles with that weapon for 1 minute. If this happens a second time the weapon or armor is lit on fire and destroyed forever.

Fire Master.                                                                                                                                    Any fire damage that Ashardalon does ignore resistance and immunity to fire damage                                                                                                                                                                  Magic Resistance. Ashardalon has advantage on saving throw against spells and other magical affects.

Regeneration. Ashardalon regains 30 hit points at the start of his turn.

Spellcasting. Ashardalon is a 20-level spellcaster. Its spellcasting ability is [Charisma] (spell save DC 24, + 17 to hit with spell attacks). The Ashardalon has following wizard spells prepared:
Cantrips (at will): firebolt, sacred flame, green flame blade, create bonfire, control flames
1st level (4 slots): Chromatic Orb, Burning Hands, Absorb elements,                                      2nd level ( 3 slots): Scorching Ray, Flaming Sphere,                                                                  3rd level ( 3 slots): Fire Ball,                                                                                                        4th level ( 3 slots): Fire Shield, Wall of fire, Raulothim's Psychic Lance                                5th level (3 slots ):                                                                                                                        6th level (2 slots): Investiture of Flame, Sun Beam                                                                      7th level (2 slots): Delayed Blast Fireball,                                                                                      8th level (1 slot): Sunburst                                                                 

Multiattack. Ashardalon may make one Bite attack, two Claw attacks, and he may use Frightful Presence.

Frightful Presence. Each creature of Ashardalon's choice that is within 120 feet of Ashardalon and aware of him must succeed on a DC 24 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 a success. If a creature succeeds it is immune to Ashardalon's frightful presence for 24 hours.

Breath Weapon. Ashardalon exhales a blast of scorching energy in a 300-foot cone. Each creature in that area must make a DC 28 Dexterity saving throw or take (28d6) fire damage and half as much on a successful save. and the creature is lit on fire it takes (2d12) fire damage at the start of its turn and has disadvantage on melee attack roles. Another creature can use its action to put out the flames. In addition any objects that are not being worn or carried in the area of the breath weapon are lit on fire as well and any creature that is in contact with them takes (3d12) fire damage at the start of its turn.

Flaming Bite. Melee Weapon Attack: +19 to hit, reach 20 ft., one target. Hit: 42 (3d12 + 15) [Piercing] damage plus: 17 [3d10] fire damage 

Claw. Melee Weapon Attack: +19 to hit, reach 15 ft., one target. Hit: 34(3d10 + 15) [Slashing] damage. If the target is a Huge or smaller creature, it is grappled ( escape DC 21) and is restrained until the grapple ends. Ashardalon can have only one creature grappled this way at a time

Tail. Melee Weapon Attack: +19 to hit, range 30 ft., cylinder. Hit: 22 (2d10 + 10) [Bludgeoning] damage. If the target is a creature, it must succeed on a DC 26 Strength saving throw or be nocked 20 feet away and have the prone condition.

Change Shape. Ashardalon magically transforms into any creature that medium or small, while retaining his game statistics (other then his its size). This transformation ends if Ashardalon is reduced to 0 hit points or uses a bonus action to end it.

Buring Stride (3/Day). Ashardalon casts Ashardalon"s Stride (detailed in Fizban's Treasury Of Dragons) at 9th level without expending a spell slot except that the area of fire damage is increased to 20 feet instead of 5.

Spell Casting. Ashardalon casts a spell that he has prepared that is 5th level or lower.

Ahardalon can take 5 legendary actions, choosing from the options below. one legendary actions can be used at a time and only one at the end of another creature's turn. Ashardalon regains any spent legendary actions at the start of his turn.

Tail attack (Costs 1 Actions). Ashardalon makes one tail attack.                               Arcane Fire Spear ( Cost 3 Actions). Ahsardalon  creats four spears of fire. Each spear hits a creature of Ashardalon's choice he can see within 120 feet of him, dealing 17 (1d10 + 10) fire damage to its target, then disappears.                                                                  Wing Attack (Cost  2 Actions). Ashardalon beats his wings. Each creature within 30 feet of Ashardalon must succeed on a DC 27 Dexterity saving throw or take 17 (2d6 + 10) Bludgeoning damage and be knocked 10 feet away and have the prone condition. Ashardalon can then fly up to half his flying speed.                                                            Cast Spell ( Cost 1 Action). Ashardalon casts one of his prepared spells expending a spell slot as usual.

If Ashardalon's Chromatic Awakening has activated in the last hour, he can use the options below as legendary actions

Ashardalon's Flaming Meteors  (Costs 5 Actions).  Ashardalon creates two flaming meteors and one of them strikes the ground at a point he can see within 120 feet of him. Each creature within 30 feet of the of the meteor must make a DC 27 dexterity saving throw or take 38 (8d8) fire damage plus 38 (8d8) Bludgeoning damage and be knocked prone. Next round he can use the second meteor with out expending any legendary actions.                                                                                                           Chromatic Flare (Cost 3 Actions). Ashardalon flares with elemental energy. Each creature of his choice in a 60-foot-radius sphere centered on him must make a DC 27 Dexterity saving throw. On a failed save, the creature takes (7d12) fire damage  and half as much on a success.                                                                                                                      Flaming Crush. ( Cost 2 Actions) Ashardalon targets a creature he is grappling. The target must succeed on a DC 27 Strength saving throw or take (4d6) fire damage and (4d6) bludgeoning damage and be thrown 30 feet in the air and fall back down taking fall damage as normal and being knocked prone. If a target succeeds it takes half as much damage and does not fall prone.