Innate Spellcasting. The Ysoaeqiy is a 20-level spellcaster. Its spellcasting ability is Charisma (spell save DC 23, +15 to hit with spell attacks). The Ysoaeqiy has following spells prepared:

Cantrips (at will): Eldritch Blast, Shocking Grasp, Thunderclap, Friends, Mind Sliver,Minor Illusion
1st level (10 slots): Chaos Bolt, Chromatic Orb, Magic Missile, Witch Bolt, Command
2nd level (9 slots): Moonbeam, Darkness
3rd level (8 slots): Fireball, Melf's Minute Meteors, Wall of Sand, Pulse Wave
4th level (5 slots): Gravity Sinkhole, Storm Sphere, Sickening Radiance, Dominate Beast
5th level (5 slots): Destructive Wave, Immolation, Arcane Hand, Wall of Force, Dominate Person
6th level (3 slots): Sunbeam, Chain Lightning
7th level (3 slots): Crown of Stars, Forcecage
8th level (3 slots): Maddening Darkness, Sunburst, Dominate Monster
9th level (2 slots): Meteor Swarm, Ravenous Void

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

Magic Weapons. Ysoaeqiy's weapon attacks are magical.

Experienced Caster. Ysoaeqiy can cast up to 3 cantrips with a single action.

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

Rapid Magic. The Ysoaeqiy Casts up to a level 5 spell slot worth of spells. ex. five 1st level  spells, two 2nd level spells and one 1st level spell, one 5th level spell, etc. Or up to three cantrips.

Multi Attack. Ysoaeqiy makes two attacks with their staff.

Staff. Melee Weapon Attack: +10 to hit, reach 10 ft., 1 target. Hit: 35 (4d12 + 10) Force damage. 

Spike Burst (Recharge 5-6). Ranged Weapon Attack: +10 to hit, range 150/600 ft., 5 targets. Hit: 35 (8d6 + 10) piercing damage.

Return to Action. Ysoaeqiy targets up to two dead creatures with in 30 ft of it and causes their bodies to melt and reform into that of eldritch horrors. These creatures return as an aberration of the DM's choice with CR equivalent to the targeted creatures previous CR or half the player level under the Ysoaeqiy's control.

Spectral Defender. Ysoaeqiy summons up to 3 specter for one round.

Abyssal Terrain. The Ysoaeqiy casts Black Tentacles and it lasts for 1 round.

Ysoaeqiy 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 mage regains spent legendary actions at the start of its turn.

Staff. The Ysoaeqiy makes an attack with it's Staff.

Crowd Control (Costs 2 Actions). The Ysoaeqiy casts Eldritch Blast without expending a spell slot.