Aryx, The Cosmic Bulwark

Legendary Resistance (4/Day). If Aryx fails a saving throw, he can choose to succeed instead.

Spellcasting. Aryx is a 19th-level spellcaster. Its spellcasting ability is Intelligence (spell save DC 24, +10 to hit with spell attacks). Aryx has following Wizard spells prepared:

7th level (4 slots): Mirage Arcane, Mental Prison, Project Image, Simulacrum

8th level (3 Slots): Illusory Dragon, Control Weather, Greater Demiplane

9th level (2 Slots): Weird, True Resurrection

Cosmic Bulwark (Recharges after a Short or Long Rest). If Aryx would be reduced to 0 hit points, his current hit point total instead resets to 500 hit points, he recharges his Breath Weapon, and he regains any expended uses of Legendary Resistance. Additionally, Aryx can now use the options in the “Mythic Actions” section for 1 hour.

Multiattack. Aryx makes one Bite attack, one Claw/Slam attack, and one Tail attack. Aryx can replace his Bite or Claw/Slam attack with a Violet Eye attack.

Bite. Melee Weapon Attack: +19 to hit, reach 20 ft., one target. Hit: 2d12 + 10 piercing damage plus 4d10 force damage.

Claw. Melee Weapon Attack: +19 to hit, reach 15 ft., one target. Hit: 2d10 + 10 slashing damage. If the target is a Huge or smaller creature, it is grappled (escape DC 20) and is restrained until this grapple ends. Aryx can have only one creature grappled this way at a time.

Slam. Melee Weapon Attack: +19 to hit, reach 15 ft., one target. Hit: 2d10 + 10 bludgeoning damage + 2d10 force damage. If the target is a Huge or smaller creature it must make a DC 26 dexterity saving throw or be knocked prone

Tail. Melee Weapon Attack: +19 to hit, reach 15 ft., one target. Hit:  2d12 + 10 bludgeoning damage. If the target is a creature, it must succeed on a DC 27 Strength saving throw or be knocked prone.

Violet Eye. Ranged Weapon Attack: +17 to hit, reach 120 ft., one target. Hit: 4d12+10 Psychic damage. The target must make a DC 24 Wisdom saving throw or become stunned for 1 turn

Frightful Presence. Each creature of Aryx's choice that is within 120 feet of the dragon and aware of it must succeed on a DC 24 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 Aryx's Frightful Presence for the next 24 hours.

Breath Weapon (Recharge 5–6). Aryx uses one of the following breath weapons

Crystalline Breath. Aryx exhales fire in a 90-foot cone. Each creature in that area must make a DC 23 Dexterity saving throw, taking 15d12 Necrotic damage on a failed save, or half as much damage on a successful one. Each creature that fails its Dexterity saving throw must make a DC 20 Constitution saving throw or become Petrified

Starfire Breath. Aryx exhales radiant, sun-like flames in a 90-foot cone. Each creature in that area must succeed on a DC 24 Strength saving throw, taking 10d20+ 30 Radiant damage on a failed save, or half as much damage on a successful one. One a failed save, each creatures has disadvantage on Strength-based attack rolls, Strength checks, and Strength saving throws 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 is killed by this attack, its body and all items are incinerated 

Psychic Step. Aryx magically teleports to an unoccupied space it can see within 60 feet of it.

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

Aryx 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. Aryx regains spent legendary actions at the start of its turn.

Detect. Aryx makes a Wisdom (Perception) check.

Tail Attack. Aryx makes a tail attack.

Planar Shift (Costs 2 Actions). Aryx moves to another plane of his choosing. Aryx may return to the material plane at will.

Wing Attack (Costs 2 Actions). Aryx beats its wings. Each creature within 15 feet of Aryx must succeed on a DC 25 Dexterity saving throw or take 17 (2d6 + 10) bludgeoning damage and be knocked prone. Aryx can then fly up to half its flying speed.

If Aryx's Cosmic Bulwark trait has activated in the last hour, he can use the options below as legendary actions.

Planar Vortex (Costs 3 Actions). All creatures within 120ft of Aryx must make a DC 20 Constitution saving throw or take 98 (4d20+20) radiant damage, or half as much on a successful save. Any creature that failed their save is knocked prone. The creature is sucked into the vortex and is moved to one of the elemental planes (decided by a d4)

Starlance (Costs 3 Actions). Aryx conjures three enormous lances of starfire that plummet to the ground at three different points he can see within 150 feet of him and then disappear. Each creature in a 20-foot-radius, 100-foot-high cylinder centered on each point must succeed on a DC 25 Dexterity saving throw or take 7d6 fire damage and 3d12 Radiant damage. A creature in the area of more than one lance is affected only once.