Atomic Mythicwyrm

Amphibious. The dragon can breathe air and water.

Legendary Resistance (11/Day, or 19/Day in Lair). If the dragon fails a saving throw, it can choose to succeed instead.

Regeneration. Every turn if the dragon has at least 1 hit point it regains 300 hit points.

Multiattack. The dragon makes nine Rend attacks. It can replace any attack with a use of (A) Spellcasting to cast [spells] or (B) Breath weapon. or (C) Bite. or (D) Swallow.

Rend. Melee Attack Roll: +22 to hit, reach 20 ft. Hit: 55 (10d8 + 10) Slashing damage plus 45 (10d8) Fire damage.

Bite. Melee Attack Roll: +23, reach 15 ft. Hit: 188 (30d10 + 23) Piercing damage plus 140 (40d6) Fire damage. Being Resistant to fire doesn’t grant Resistance to this Fire damage.

Swallow. Dexterity Saving Throw: DC 45, one creature Grappled by the dragon (it can have up to nine creatures swallowed at a time). Failure: 165 (30d8 + 30) Piercing damage. A swallowed creature has the Restrained condition, has Total Cover against attacks and other effects outside the dragon, and takes 105 (30d6) Acid, and takes 105 (30d6) fire damage at the start of each of its turns.

Fire Breath (Recharge 2–6). Dexterity Saving Throw: DC 44, each creature in a 190-foot Cone. Failure: 325 (50d12) Fire damage. Success: Half damage.

Spellcasting. The dragon casts two of the following spells, requiring no Material components and using Charisma as the spellcasting ability (spell save DC 44, +16 to hit with spell attacks):

At Will: Detect Magic, Guiding Bolt (level 9 version), Shapechange (Beast or Humanoid form only, no Temporary Hit Points gained from the spell, and no Concentration or Temporary Hit Points required to maintain the spell),Time Stop,Fear
6/Day Each: Flame Strike (level 9 version), Word of Recall, Zone of Truth,wish,Meteor Swarm

Weakening Breath. Strength Saving Throw: DC 44, each creature that isn’t currently affected by this breath in a 190-foot Cone. Failure: The target has Disadvantage on all-based D20 Tests and subtracts 130 (20d12) from its damage rolls. The target must succeed on a DC 44 Constitution saving throw or be stunned for 10 minutes . It repeats the save at the end of each of its turns, ending the effect on itself on a success. After 2 hours, it succeeds automatically.

Spellcasting. The dragon casts two of the following spells, requiring no Material components and using Charisma as the spellcasting ability (spell save DC 44, +16 to hit with spell attacks)

Bite. Melee Attack Roll: +23, reach 15 ft. Hit: 188 (30d10 + 23) Piercing damage plus 140 (40d6) Fire damage. Being Resistant to fire doesn’t grant Resistance to this Fire damage.

Swallow. Dexterity Saving Throw: DC 45, one creature Grappled by the dragon (it can have up to nine creatures swallowed at a time). Failure: 165 (30d8 + 30) Piercing damage. A swallowed creature has the Restrained condition, has Total Cover against attacks and other effects outside the dragon, and takes 105 (30d6) Acid, and takes 105 (30d6) fire damage at the start of each of its turns.

Rend. Melee Attack Roll: +22 to hit, reach 20 ft. Hit: 55 (10d8 + 10) Slashing damage plus 45 (10d8) Fire damage.

Bite. Melee Attack Roll: +23, reach 15 ft. Hit: 188 (30d10 + 23) Piercing damage plus 140 (40d6) Fire damage. Being Resistant to fire doesn’t grant Resistance to this Fire damage.

Legendary Action Uses: 5 (7 in Lair). Immediately after another creature’s turn, the dragon can expend a use to take one of the following actions. The dragon regains all expended uses at the start of each of its turns.

Banish. Charisma Saving Throw: DC 24, one creature the dragon can see within 120 feet. Failure: 24 (7d6) Force damage, and the target has the Incapacitated condition and is transported to a harmless demiplane until the start of the dragon’s next turn, at which point it reappears in an unoccupied space of the dragon’s choice within 120 feet of the dragon. Failure or Success: The dragon can’t take this action again until the start of its next turn.

Guiding Light. The dragon uses Spellcasting to cast Guiding Bolt (level 9 version).

Pounce. The dragon moves up to half its Speed, and it makes five Rend attack.

Bite. Melee Attack Roll: +23, reach 15 ft. Hit: 188 (30d10 + 23) Piercing damage plus 140 (40d6) Fire damage. Being Resistant to fire doesn’t grant Resistance to this Fire damage.

Swallow. Dexterity Saving Throw: DC 45, one creature Grappled by the dragon (it can have up to nine creatures swallowed at a time). Failure: 165 (30d8 + 30) Piercing damage. A swallowed creature has the Restrained condition, has Total Cover against attacks and other effects outside the dragon, and takes 105 (30d6) Acid, and takes 105 (30d6) fire damage at the start of each of its turns.

Mythic Action Uses: 4 (5 in Lair). Immediately after another creature’s turn, the dragon can expend a use to take one of the following actions. The dragon regains all expended uses at the start of each of its turns.

Spellcasting. The dragon casts two of the following spells, requiring no Material components and using Charisma as the spellcasting ability (spell save DC 44, +16 to hit with spell attacks):

Plasma Breath (Recharge 2–6). Dexterity Saving Throw: DC 44, each creature in a 190-foot Cone. Failure: 975 (150d12) Fire damage this negates all ways of reducing damage. Success: -50 damage.