Crimson Greatwyrm

Legendary Resistance. 6/day, if the Crimson Greatwyrm fails a saving throw, he can choose to succeed instead.

False Form. If the Crimson Greatwyrm is killed he returns to his true form rather than dying

Spell Immunity. He is immune to spells less than 4th level.

Summoning Phase. Once he is reduced to 100 HP, he retreats backwards and surrounds himself with flames, from there he will spawn waves of Fire Elementals and Battleforce Angels. He will regenerate 30 HP per round, once he reaches 400 HP the phase ends. He cannot use actions or legendary actions during this phase.

Crimson Vengeance.  After the Summoning Phase is over, he enters his third phase, increasing his size to Titanic which adds an extra 30 ft to all of his attacks. He gains +10 to his strength score and every melee attack he makes deals an extra 3d8 fire damage if it doesn't already. He also becomes immune to all damage dealt to him by a creature smaller than Gargantuan size. He also gains access to the Mythical Actions table for 1 hour.

Spellcasting. The Crimson Greatwyrm is a 15-level spellcaster. Its spellcasting ability is charisma (spell save DC 24, +24 to hit with spell attacks). The Crimson Greatwyrm has the following sorcerer spells prepared:
2nd level (3 slots): Cloud of Daggers (5th level)
3rd level (2 slots): Lightning Bolt (4th level), Erupting Earth (4th level)
6th level (2 slots): Chain Lightning
7th level (1 slot): Firestorm, Plane Shift.

Multiattack. The Crimson Wyrm makes two claw attacks, a spell and a flame surge attack. Or, one bite, one tail and one wing attack.

Claw Attack. Melee Weapon Attack: +19 to hit, reach 40 ft., 1 target. Hit (6d6 + 10) Slashing damage.

Bite. Melee Weapon Attack: +19 to hit, reach 20 ft., 1 target. Hit (3d10+10) Piercing damage and (2d8) Fire damage

Tail. Melee Weapon Attack: +19, reach 75 ft., 1 target. Hit (5d8+10) Bludgeoning damage.

Wing. Ranged Weapon Attack: DC 27 Dex, reach 200 ft radius., multiple targets. Hit (5d12) Force. any structures in this area are also affected

Flame Surge. Ranged Attack: DC 27 Dex, range 100 ft radius., multiple targets.  Hit (6d12) Fire damage and (3d12) Radiant damage.

Crimson Breath. Ranged Attack: DC 27 Dex, range 500 ft cone., multiple targets. Hit (26d6) Fire damage, (3d12) Radiant damage and (3d12) Necrotic damage. Any flammable material in said cone that isn't being worn or wielded is incinerated.

Legendary Actions: The Crimson Greatwyrm may use a legendary action at the end of another creature's turn or in place of his own Turn. He has 5 and regains all expended uses at the start of his turn.

Claw (Costs 1 Action). Melee Weapon Attack: He makes 2 claw attacks.

Flame Surge (Costs 1 Action). Ranged Magic Attack: He uses his Flame Surge attack.

Mana Rain (Costs 2 Actions). Ranged Spell Attack: He ignites the natural Mana in the area, causing random and chaotic events to raze the surrounding area, requires him to still have a 6th level spell slot. DC 24 Con, 300 ft sphere centred around himself., multiple targets. Hit (12d8) damage. Roll a D10, the roll determines the damage type: 
1. Radiant, 2. Necrotic, 3. Cold, 4. Fire, 5. Lightning, 6. Poison, 7. Thunder, 8. Acid, 9. Force, 10. Psychic.

Mythic Actions:  The Crimson Greatwyrm can use the following moves as Legendary Actions, only during his third phase.

Crimson Armour (Costs 3 Actions). The Crimson Greatwyrm surrounds himself in shimmering red magic, the next two attacks made against him will deal half their damage and anyone who hits him will take 25 Force Damage.

Crimson Judgement (Costs 5 Actions). Melee Spell Attack: The Crimson Greatwyrm unleashes a flare of raw power. DC 27 Str save, 150 ft radius around him., Multiple targets. Hit (40d6) Fire damage, (5d6) Radiant damage and (5d6) Force damage.