Agonizing Blast.  When Void casts Eldritch Blast, it can add their Charisma modifier to the damage it deals on a hit.

Amphibious. Void can breathe in air and water.

Limited Magic Immunity. Unless Void wishes to be affected, Void is immune to spells of 3rd level or lower. They have advantage on saving throws against all other spells and magical effects.

Legendary Resistance (4/day). If Void fails a saving throw, they can choose to succeed it instead.

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

Spellcasting. Void is a 21st level spellcaster. His spellcasting ability is Charisma (spell save DC 25, +17 to hit with spell attacks). Void knows the following spells and can cast them requiring no material components.

Cantrips (At-Will): eldritch blast, shape water, thaumaturgy, chill touch

1st Level Spells (4 slots): cause fear, armor of agathys, shield

2nd Level Spells (3 slots): darkness, hold person, shatter, suggestion

3rd Level Spells (3 slots): counterspell, dispel magic, fear, wall of water

4th Level Spells (3 slots): banishment, evard's black tentacles, sickening radiance, watery sphere

5th Level Spells (3 slots): dream, hold monster, scrying

6th Level Spells (2 slots): disintegrate, flesh to stone

7th Level Spells (2 slots): finger of death, forcecage

8th Level Spells (2 slots): dominate monster, feeblemind, power word stun, tsunami

9th Level Spells (2 slots): meteor swarm, storm of vengeance, wish

Sunlight Sensitivity. Void has disadvantage on attack rolls and perception checks involving sight when Void, the target of its attack, or whatever it is trying to see is in direct sunlight.

Multiattack. Void can use his mind blast if it is recharged. He then makes a Sword of the Void attack and Psionic Tentacle attack, or a Psionic Slam attack and casts one spell that takes an action or bonus action to cast. 

Sword of the Void. Melee Weapon Attack: +19 to hit, reach 20 ft., one target. Hit: 46 (8d8 + 10) slashing damage, and the target must succeed a DC 23 Constitution saving throw or have disadvantage on all Strength, Dexterity, and Constitution saving throws for 1 minute.  A creature can repeat this saving throw again at the end of each of it's turning, ending the effect on a success.

Psionic Tentacle. Melee Weapon Attack: +15 to hit, reach 120 ft., one target. Hit: 35 (5d10 + 6) psychic damage and the target is grappled (escape DC 23).  While the grapple persists, the target is also restrained.  Void has 6 Psionic Tentacles that can be used to attack.

Psionic Slam.  Any creature grappled by one of Void's Psionic Tentacle is raised 30 feet into the air and slammed into the ground for 35 (5d10+6) bludgeoning damage.

Eldritch Blast (Cantrip). Ranged Spell Attack: +17 to hit, range 120 ft., one creature. Hit:13 (1d10+8) force damage.

Mind Blast (Recharge 6). Void magically emits psychic energy in a 100-foot cone. Each creature in that area must succeed on a DC 23 Intelligence saving throw or take 86 (12d12+6) psychic damage and be stunned until the end of Void's next turn.

Void can take 3 legendary actions, choosing from the options below. Only one legendary option can be used at a time and only at the end of another creature's turn. Void regains spent legendary actions at the beginning of his turn.

Slam. Melee Weapon Attack: +19 to hit, reach 15 ft., one target. Hit: 24 (4d8 + 10) bludgeoning damage.

Recharge (Only usable once per round). Void can attempt to recharge its Mind Blast ability if it is currently uncharged.

Cantrip.  Void casts a cantrip.

Spell (Costs 2 Actions).  Void casts a spell.

Buffeting Wings (Costs 2 Actions). Every creature of large or smaller size within 30 feet of Void must succeed on a DC 27 Strength saving throw or be pushed 20 ft. away from him.  Void can then move up to half its movement speed in any direction.