Colossal. Ziridix is immune to forced movement, does not provoke opportunity attacks, and cannot make opportunity attacks.

In addition, whenever Ziridiax hits with a melee attack, each creature within 20 feet of the attack's target (other than the target or Ziridiax himself) must succeed on a DC 23 Dexterity saving throw or take half the attack's damage.

Empowered Attacks. Ziridiax's melee attacks count as magical for the purpose of overcoming resistance and immunity to nonmagical attacks. 

Immutable Form. Ziridiax is immune to any spell or effect that would alter his form.

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

In addition, Ziridiax has resistance to all damage. If a creature strikes Ziridiax with an Ancient Dragon Ballista Bolt, he loses resistance to a damage type of the creatures choosing.

Limited Magic Immunity. Unless he wishes to be affected, Ziridiax is immune to spells of 3rd level or lower that target him or include him in their area of effect. He has advantage on saving throws against all other spells and magical effects.

Heightened Concentration. Ziridiax may concentrate on up to 3 separate spells and may choose which spell to stop concentrating on should he fail a concentration saving throw. Ziridiax also has advantage on saving throws against being incapacitated and bring knocked unconcious.

Spellcasting. Ziridiax is a 20th level spellcaster. His spellcasting ability is charisma (spell save DC 23, +15 to hit with spell attacks). He can cast the following spells, requiring no material components:

• Cantrips (at will): all
• 1st level (at will): Bane, Chromatic Orb, Command, Detect Magic, Identify, Inflict Wounds
• 2nd level (at will): Blindness/Deafness, Dragon's Breath, Knock, Misty Step, Ray of Enfeeblement, Suggestion
• 3rd level (at will): Bestow Curse, Dispel Magic, Incite Greed, Remove Curse, Spirit Guardians, Fireball, Lightning Bolt
• 4th level (at will): Banishment, Death Ward, Elemental Bane, Leomund's Secret Chest
• 5th level (3 slots): Antilife Shell, Circle of Power, Contagion, Legend Lore, Scrying, Cone of Cold
• 6th level (3 slots): Circle of Death, Disintegrate, Mass Suggestion, Scatter, Arcane Gate, Chain Lightning, Eyebite, Freezing Sphere, Mental Prison
• 7th level (3 slots): Divine Word, Finger of Death, Plane Shift, Prismatic Spray, Fire Storm, Whirlwind
• 8th level (2 slots): Earthquake, Feeblemind, Illusory Dragon, Tsunami
• 9th level (2 slots): Invulnerability, Meteor Swarm, Psychic Scream, Power Word Kill, Storm of Vengeance, Abi-Dalzim's Horrid Wilting, Gate

Multiattack. Ziridiax can use Frightful Presence, and if he was flying at the start of his turn, he can use Crushing Landing. He then makes three attacks: one with chromatic bite, one with his claws, and one with his tail stinger. He may also replace one attack with a spell. 

Chromatic Bite. Melee Weapon Attack: +12 to hit, reach 15 ft., one target. Hit: 23 (2d12 + 12) piercing damage plus 7 (2d6) radiant damage, 7 (2d6) cold damage, 7 (2d6) fire damage, 7 (2d6) lightning damage, 7 (2d6) poison damage, and 7 (2d6) necrotic damage.

Claw. Melee Weapon Attack: +12 to hit, reach 10 ft., one target. Hit: 32 (4d10 + 10) slashing damage. If the target is a creature, it must succeed on a DC 23 strength saving throw or be flung 60 ft away from Ziridiax, landing prone. 

Tail Stinger. Melee Weapon Attack: +12 to hit, reach 20 ft., one target. Hit: 36 (4d12 + 10) bludgeoning damage. If the target is a creature, it must succeed on a DC 23 constitution saving throw or be poisoned until the end of its next turn. While poisoned in this way, a creature is under the effects of a Slow spell.

Frightful Presence. Each creature of Ziridiax's choice that is within 120 feet of the dragon and aware of it must succeed on a DC 23 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 itself on a success. If a creature's saving throw is successful or the effect ends for it, the creature is immune to Ziridiax's Frightful Presence for the next 24 hours.

Crushing Landing. If Ziridiax is flying, he can attempt to land forcefully in a space of his choosing, regardless of whether that space is occupied by other creatures. When he does so, his movement speed is reduced to 0 until the end of his turn, and each creature in Ziridiax's space or within 60 ft of him, other than Ziridiax himself, must succeed on a DC 23 dexterity saving throw or take 39 (6d12) bludgeoning damage and be knocked prone.

Chromatic Breath (Recharge 5–6). Ziridiax exhales multicolored flames in a 300-foot cone. Each creature in that area must make a DC 23 Dexterity saving throw. On a failed save, the creature takes 71 (11d12) force damage. On a successful save, the creature takes half as much damage. On a failed save, each creature must succeed on a DC 23 saving throw or succumb to one of the conditions below (chosen by Ziridiax) until the end of their next turn:  

• Desiccation: constitution saving throw. On failure, the creature has disadvantage on strength saving throws and strength weapons deal half damage.
• Singulation: strength saving throw. On failure, the creature's movement speed is reduced to 0.
• Disorientation: intelligence saving throw. On failure, the creature's attack rolls are reduced by 1d8.
• Irradiation: dexterity saving throw. On failure, the creature is blinded.
• Horrification: wisdom saving throw. On failure, the creature is frightened. 
• Demoralization: charisma saving throw. On failure, the creature has disadvantage on attack rolls and cannot receive healing.

Quicken Spell (5/day). Ziridiax casts one of his spells of 6th level or lower with a casting time of 1 action.

Consume. Ziridiax may consume a creature he has successfully hit with chromatic bite. A consumed creature is immune to all damage and effects from outside of Ziridiax. In addition, the creature takes 22 (4d10) acid damage at the start of each of their turns as they are slowly digested. The creature will be regurgitated if they deal 50 damage to Ziridiax in a single turn or if Ziridiax takes 100 damage from a creature outside in a single turn.  

Divine Denial. When a creature within 300 ft of Ziridiax that he can see casts a spell, he can use his reaction to force the creature to make a DC 23 wisdom saving throw. On a failure, the spell fails. Creatures immune to charm are immune to this effect. Clerics of good-aligned deities may use their Divine Intervention to prevent Ziridiax from using this feature.

Ziridiax can take 5 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. Ziridiax regains spent legendary actions at the start of his turn.

Detect. Ziridiax makes a Wisdom (Perception) check.

Tail Attack. Ziridiax makes a tail stinger attack.

Wing Attack (Costs 2 Actions). Ziridiax beats his wings. Each creature within 30 feet of the dragon must succeed on a DC 23 Dexterity saving throw or take 28 (4d8 + 10) bludgeoning damage and be knocked prone. The dragon can then fly up to half its flying speed.

Cast a Spell (Costs 2 Actions). Ziridiax casts one of his spells of 6th level or lower.

Breath Weapon (Costs 3 Actions). Ziridiax uses uses an ancient dragon breath weapon. He exhales in a 300 ft cone. Each creature in that area must succeed on a DC 23 saving throw based on the breath chosen or succumb to its condition and take 71 (11d12) damage. A creature takes half damage on a successful saving throw and receives no condition. 

• Desiccation (cold): constitution saving throw. On failure, the creature has disadvantage on strength saving throws and strength weapons deal half damage.
• Singulation (lightning): strength saving throw. On failure, the creature's movement speed is reduced to 0.
• Disorientation (poison): intelligence saving throw. On failure, the creature's attack rolls are reduced by 1d8.
• Irradiation (radiant): dexterity saving throw. On failure, the creature is blinded.
• Horrification (psychic): wisdom saving throw. On failure, the creature is frightened. 
• Demoralization (necrotic): charisma saving throw. On failure, the creature has disadvantage on attack rolls and cannot receive healing.

Upon reaching 50% health, Ziridiax gain access to 2 mythic actions, choosing from the options below in addition to his legendary actions. Only one mythic action option can be used at a time and only at the end of another creature's turn. Ziridiax regains spent mythic actions at the start of his turn.

Chromatic Flare (Costs 2 Actions). Ziridiax flares with elemental energy. Each creature of Ziridiax's choice in a 60-foot-radius sphere centered on him must make a DC 23 Dexterity saving throw. On a failed save, the creature takes 71 (11d12) damage of a type chosen by Ziridiax: psychic, cold, fire, lightning, poison, radiant, or necrotic. On a successful save, the creature takes half as much damage.

Shackles of Pain (Costs 1 Actions). Ziridiax channels power from his master in the form of ethereal chains. These chains bind Ziridiax to a creature of his choosing, tormenting their body and mind. A shackled creature receives half of all damage dealt to Ziridiax. Shackles of Pain may only be broken by a successful DC 23 wisdom saving throw, a melee weapon attack dealing at least 30 damage, or if the target reaches 0 hitpoints. If a shackled target reaches 0 hitpoints due to damage from Shackles of Pain, the target dies.  If a creature breaks Shackles of Pain, Ziridiax gains one cast of Power Word Kill.