Ancient Gold Greatwyrm

Metallic Awakening. When the Ancient Gold Greatwyrm would drop to 0 hit points, it instead regains 450 hit points and may now take actions from the Mythic Actions section. Additionally, it regains all expended legendary actions and resistances.

World Ending Presence. All creatures within 240 feet of the Greatwyrm that it chooses must make a DC 24 Wisdom Save. Succeeding against this save renders a creature immune to this effect for 24 hours. Creatures that fail this save take one of the following effects (Greatwyrm chooses): (A) become paralyzed for one minute, (B) be compelled to expend all possible resources to flee as far away from the Greatwyrm as possible, (C) become Frightened and Confused for 5 minutes

Spellcasting. The Greatwyrm is a 20-level spellcaster. Its spellcasting ability is Charisma (spell save DC 24, +16 to hit with spell attacks). It has following [class name] spells prepared:
Cantrips (at will): Bless, Bane, Chain Lightning, Disintegrate, Hold Person (5th Level), Guiding Bolt (5th Level), Greater Invisibility, Blade Barrier, Heroes Feast, Resurrection

5/Day- Anti-magic Field, Power Word Stun, Control Weather 

3/Day- Meteor Swarm, Mass Heal, Time Stop

Legendary Resistances. The Greatwyrm has 7 legendary resistances/day. 

Multiattack. The Greatwyrm makes 3 attacks, choosing from the following options.

Rend. +16 to hit, reach 15 ft., 1 target. Hit: 21 (2d10+10) magical slashing damage + 21 (2d10+10) Force damage.

Bite. +16 to hit, range 20 ft., 1 target. Hit: 35 (4d10 + 13) magical piercing damage. Target must make a DC 24 Strength save or be knocked prone.

Tail. +16 to hit, range 50ft, 1 target. Hit: 21 (2d10+10) force damage.

Cast Spell. The Greatwyrm casts 1 spell from its spell list.

Breathe Weapon (recharge on 4-6). The Greatwyrm breathes in a 200 ft. line, causing all creatures to make a DC 24 dexterity saving thrown or take 84 (13d12) fire damage and 42 (7d12) radiant damage (half as much on a success). 

The Greatwyrm has 5 legendary actions/round. 

• Attack. The Greatwyrm makes one Claw or Tail attack.

• Wing Attack (Costs 2 Actions). The Greatwyrm beats its wings. Each creature within 30 feet of it must succeed on a DC 26 Dexterity saving throw or take 17 (2d6 + 10) bludgeoning damage and be knocked prone. The Greatwyrm can then fly up to half its flying speed.

• Teleport (Costs 3 Actions). The Greatwyrm vanishes and reappears in any unoccupied space it can see within 500 feet. 

Breathe Weapon(Mythic recharge 6)- The Greatwyrm uses its breathe weapon.

Spellcasting (Costs 2 Actions)- The Greatwyrm casts a spell from its spell list.