Aryx, Planar Devourer

Amphibious. The dragon can breathe air and water.

Advanced Probing Telepathy. If a creature communicates telepathically with Aryx, he learns almost everything about the creature

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

Magic Resistance. Aryx has advantage on saving throws against spells and other magical effects

Crystalline Shell. Any time Aryx is targeted by a magic missile spell, a line spell, or a spell that requires a ranged attack roll, roll a d6. On a 1 to 3, the spell hits. On a 4 to 5 Aryx is unaffected. On a 6, Aryx is unaffected, and the effect is reflected back at the caster as though it originated from Aryx, turning the caster into the target.

Spellcasting. Aryx is a 19th-level spellcaster. Its spellcasting ability is Intelligence (spell save DC 23, +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 

Multiattack. Aryx can use its Frightful Presence. It then makes three attacks: one with its bite and two with its tail and/or one with violet eye, tail and bite

Bite. Melee Weapon Attack: +17 to hit, reach 15 ft., one target. Hit: 21 (2d10 + 10) piercing damage.

Violet Eye. Melee Weapon Attack: +17 to hit, reach 10 ft., one target. Hit: 26 (2d10 + 10 Peircing) + 15 (3d6+5 Psychic)

Tail. Melee Weapon Attack: +18 to hit, reach 20 ft., one target. Hit: 19 (2d8 + 10) bludgeoning damage.

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 Weapons (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 24 Dexterity saving throw, taking 130 (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 22 Constitution saving throw or become Petrified

Ionising Breath. Aryx exhales gas in a 90-foot cone. Each creature in that area must succeed on a DC 24 Strength saving throw, taking 190 (10d20+ 30) Fire 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 liquified

Change Shape. Aryx magically polymorphs into a humanoid or beast that has a challenge rating no higher than its own, or back into its true form. It reverts to its true form if it dies. Any equipment it is wearing or carrying is absorbed or borne by the new form (Aryx's).

In a new form, the dragon retains its alignment, hit points, Hit Dice, ability to speak, proficiencies, Legendary Resistance, lair actions, and Intelligence, Wisdom, and Charisma scores, as well as this action. Its statistics and capabilities are otherwise replaced by those of the new form, except any class features or legendary actions of that form.

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. The dragon regains spent legendary actions at the start of its turn.

Detect. The dragon makes a Wisdom (Perception) check.

Tail Attack. The dragon makes a tail attack.

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.

Aryx can take 2 Mythic actions, choosing from the options below. Only one legendary action option can be used at a time.

Planar Vortex. 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)

Planar Shift. Aryx moves to another plane of his choosing