Commander Aruum the falling flame

Trait 1 - Spell casting 

Spellcasting. Commander Aruum is a 20th-level spellcaster. Its spellcasting ability is intelligence (spell save DC 20, +12 to hit with spell attacks). Commander Aruum has the  following [wizard] spells prepared:

Cantrips (at will): Mage Hand, Fire Bolt, Minor Illusion, Prestidigitation
1st level (4 slots): Shield, Magic Missile, Absorb Elements, Detect Magic
2nd level (3 slots): Mirror Image, Misty Step, Hold Person
3rd level (3 slots): Counterspell, Fireball, Dispel Magic
4th level (3 slots): Greater Invisibility, Banishment, Dimension Door
5th level (3 slots): Wall of Force, Hold Monster, Telekinesis
6th level (2 slots): Chain Lightning, Globe of Invulnerability
7th level (2 slots): Forcecage, Power Word Pain
8th level (1 slot): Power Word Stun
9th level (1 slot): Power Word Kill

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

Magnetic Field. Metal weapons and armor are subtly pulled toward Aruun. Melee attackers using metal weapons must succeed on a DC 15 Strength saving throw or make their attack with disadvantage.

Mythic Trait – Starborn Rebirth. When Aruun is reduced to 0 HP, he does not die. Instead, he erupts with stellar energy and returns to combat in his mythic form with 200 hit points. His powers intensify, and new actions and effects become available. While in mythic form, he emits a 60-foot gravitational aura that halves movement speed and suppresses flying unless a creature succeeds on a DC 15 Constitution saving throw at the start of its turn.

Gravity Cleave. Melee Weapon Attack: +13 to hit, reach 10 ft., one target.
Hit: 27 (4d10) force damage plus 10 (3d6) fire damage. The target must succeed on a DC 20 Strength saving throw or be pulled 10 feet toward Aruun and knocked prone.

Arcane Implosion (Recharge 5–6). Aruun causes space to rupture in a 30-foot-radius sphere centered on himself. Each creature in that area must make a DC 20 Constitution saving throw. On a failed save, a creature takes 66 (12d10) force damage and is pushed 20 feet directly away from Aruun. On a successful save, the creature takes half damage and isn’t pushed.

Flare Nova (1/Day). Aruun releases an explosion of solar flame. Each creature in a 60-foot radius must make a DC 20 Dexterity saving throw. A creature takes 55 (10d10) radiant damage and 55 (10d10) fire damage on a failed save, or half as much on a successful one.

Stellar Edge (Melee Weapon Attack). Melee Weapon Attack: +13 to hit, reach 10 ft., one target. Hit: 28 (3d10 + 6) radiant damage plus 14 (3d10) force damage. Once per turn on a hit, Aruun can teleport to an unoccupied space within 30 feet of the target.
On a critical hit, the blade erupts in a burst of stellar energy. All other creatures within 10 feet must make a DC 19 Dexterity saving throw, taking 22 (4d10) force damage on a failure, or half on a success.

Starstep. Aruun teleports up to 30 feet to an unoccupied space he can see. The space he leaves behind erupts with gravitational energy. Until the start of his next turn, that space counts as difficult terrain, and any creature that enters it or starts its turn there must succeed on a DC 18 Strength saving throw or be pulled 10 feet toward the center and knocked prone.

Temporal Warp (Recharge 4–6). When a creature Aruun can see casts a spell or makes an attack against him, he can twist time slightly to roll a d6. On a 4–6, the triggering action fails or misses as if it never happened. This does not use a spell slot or require concentration.

Aruun 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. Aruun regains spent legendary actions at the start of his turn.

Meteor Spear. Aruun launches a blazing spear of cosmic flame at a target within 120 feet. Hit: 21 (6d6) fire damage. If the target fails a DC 18 Dexterity save, the spear explodes in a 10-foot radius for 10 (3d6) additional fire damage.

Graviton Shift. Aruun alters the local gravity in a 20-foot cube within 60 feet. Until the start of his next turn, the area becomes difficult terrain and flying creatures must succeed on a DC 18 Constitution saving throw or fall.

Rip Through Reality (Costs 2 Actions). Aruun teleports up to 60 feet and immediately casts a cantrip or 1st-level spell without verbal or somatic components.

When Aruun enters his mythic form, he gains access to the following mythic actions for 1 minute or until reduced to 0 HP again.

Solar Annihilation (Costs 2 Actions). Aruun channels a collapsing star in a 60-foot cone. Each creature must make a DC 21 Dexterity saving throw, taking 55 (10d10) radiant damage and becoming blinded for 1 minute on a failure. A successful save halves the damage and prevents the blindness.

Astral Lock (Costs 1 Action). Aruun targets one creature he can see within 60 feet. The target must succeed on a DC 20 Constitution saving throw or be restrained by bands of force for 1 minute. At the end of each of its turns, the creature can repeat the saving throw.

Starfire Chain. Aruun hurls arcs of starlight at up to three targets he can see within 60 feet. Each target takes 22 (4d10) force damage and must make a DC 18 Strength saving throw or be pulled 10 feet closer to Aruun.