Blackphire

Ice Walk. Blackphire 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 him extra movement.

Achromatic Wrath (Recharges after a Short or Long Rest). If Blachphire would be reduced to 0 hit points, his current hit point total instead resets to 500 hit points, he recharges his Achromatic ice, and he regains any expended uses of Legendary Resistance. Additionally, he can now use the options in the "Mythic Actions" section for 1 hour. Award a party an additional 145,000 XP (265,000 XP total) for defeating Blackphire after his Achromatic Wrath activates.

Legendary Resistance (3/Day). If Blackphire fails a saving throw, he can choose to succeed instead.

Multiattack. Blackphire makes three attacks: one with his bite and two with his claws.

Bite. Melee Weapon Attack: +14 to hit, reach 15 ft., one target. Hit: 19 (2d10 + 8) piercing damage plus 9 (2d8) cold damage.

Claw. Melee Weapon Attack: +14 to hit, reach 10 ft., one target. Hit: 15 (2d6 + 8) slashing damage.

Tail. Melee Weapon Attack: +14 to hit, reach 20 ft., one target. Hit: 17 (2d8 + 8) bludgeoning damage.

Cold Necrotic Breath (Recharge 5–6). Blackphire exhales an icy blast in a 90-foot cone that's 10 feet wide. Each creature in that area must make a DC 22 Constitution saving throw, taking 72 (16d8) cold or necrotic, (Blackphire's choice) damage on a failed save, or half as much damage on a successful one.

Acid Breath (Recharge 5–6). Blackphire exhales acid in a 90-foot line that is 10 feet wide. Each creature in that line must make a DC 22 Dexterity saving throw, taking 67 (15d8) acid damage on a failed save, or half as much damage on a successful one.

 Change Shape. Blackphire magically transforms into any creature that is Medium or Small, while retaining his game statistics (other than its size). This transformation ends if the dragon is reduced to 0 hit points or uses a bonus action to end it.

Blackphire 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. Blackphire regains spent legendary actions at the start of his turn.

Detect. Blackphire makes a Wisdom (Perception) check.

Tail Attack. Blackphire makes a tail attack.

Wing Attack (Costs 2 Actions). Blackphire beats its wings. Each creature within 15 feet of him must succeed on a DC 22 Dexterity saving throw or take 15 (2d6 + 8) bludgeoning damage and be knocked prone. Blackphire can then fly up to half his flying speed.

 If Blackphire's Achromatic Wrath trait has activated in the last hour, he can use the options below as legendary actions.

Desolation breath Achromatic Flare (Costs 3 Actions). Blackphire flares with elemental energy. Each creature of Blackphire's 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 39 (6d12) damage of a type chosen by Blackphire: \cold, or necrotic, or acidic. On a successful save, the creature takes half as much damage.

Desolations Song (costs 3 actions) Blackphire, seeing that the foes he faces are proving to be more meddlesome then he wishes, he blasts out three shots an acidic ball, a icy cone, and a necrotic mucus, each doing 8d6 damage of their respective types. each blast affects a 40 foot radius of his choosing.
The draw back is that the normal recharge for his breath weapons are doubled as he used both at once and will need an extra 3 turns to recoup