Cirathium the Great

Defecting Force. Cirathium always gains a +4 to all saving throws and AC this effect can only be deactivated if Cirathium enters a 10th level anti magic zone. 

Unusual Arcane Nature. Cirathium doesn't require food or drink, Cirathium also counts as having the magical weapons trait when it comes to overpowering damage resistances and immunities. 

Immunity to light and Prismatics. Cirathium is immune to all spells or effects involving or using light or prismatic effects. 

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

Legendary Resistance (5/Day). Cirathium fails a saving throw, he can choose to succeed instead.

Innate Spellcasting. Cirathium's innate spellcasting ability is Charisma (DC 27, +19 to hit with spell attacks). Cirathium can innately cast the following spells, requiring no components:

At Will: Hypnotic pattern

3/day: Prismatic wall, Sunbeam, Sunburst 

Wizard Spellcasting. Cirathium is a 25-level spellcaster. Its spellcasting ability is Intelligence (spell save DC 27, +19 to hit with spell attacks). The [monster name] has following [class Wizard spells prepared:
Cantrips (at will): Mold earth, Friends, Mage hand, Prestidigitation
1st level (4 slots): Catapult, Identify, Shield, Sleep
2nd level (3 slots): Blur, Detect Thoughts, Enlarge/Reduce, Magic Mouth
3rd level (3 slots): Counterspell, Dispel Magic, Haste, Remove curse 
4th level (3 slots): Banishment, Fabricate, Greater Invisibility, Leomond's secret chest  
5th level (3 slots): Conjure elemental, Dream, Legend lore, Telekinesis
6th level (3 slots): Drawmji's instant summons, Fizban's platinum shield, Mass suggestion 
7th level (2 slots): Create Magen, Plane shift, Teleport 
8th level (2 slots): Demiplane, Illusory dragon, Telepathy
9th level (2 slots): Foresight, Time stop, Wish
10th level (1 slots): Kinetic Control , Create Mythal  

Sorcerer Spellcasting. Cirathium is a 25th-level spellcaster. Its spellcasting ability is Charisma (spell save DC 27, +19 to hit with spell attacks). Cirathium has following Sorcerer spells prepared:
Cantrips (at will): Chill Touch, Mind sliver, Shocking grasp, Thunderclap
1st level (4 slots): Chromatic orb, Detect magic
2nd level (3 slots): Magic Weapon, Shatter
3rd level (3 slots): Fireball, Vampiric touch
4th level (3 slots): Raulothim's Psychic Lance, Sickening Radiance
5th level (3 slots): Synaptic static, Cone of cold
6th level (3 slots): Disintegrate, Mental prison
7th level (2 slots): Crown of Stars, Finger of Death
8th level (2 slots): Earthquake, Power word stun
9th level (2 slots): Meteor swarm
10th level (1 slots): Necromantic Singularity 

Multi attack. Cirathium uses its frightful prescience then makes a bite attack and two claw attacks. 

Bite. Melee Weapon Attack: +19 to hit, reach 20 ft., 1 target. Hit: 26 (3d10 + 10) piercing damage plus 26 (4d12) force damage. 

Claw. Melee Weapon Attack: +19 to hit, reach 15 ft., 1 target. Hit: 23 (3d8 + 10) slashing damage. If the target is a Gargantuan or smaller creature, it is grappled (escape DC 20) and is restrained until this grapple ends. Cirathium can have only one creature grappled this way at a time.

Tail. Melee Weapon Attack: +19 to hit, reach 25 ft., 1 target. Hit: 23 (3d8 + 10) bludgeoning damage. If the target is a creature, it must succeed on a DC 27 Strength saving throw or be knocked prone.

Frightful Presence. Each creature of Cirathium's choice that is within 120 feet of Cirathium and aware of him must succeed on a DC 27 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 Cirathium's Frightful Presence for the next 24 hours.

Prismatic breath (Recharges 5/6). Cirathium exhales prismatic lights in a 300ft cone, each creature within range must make a DC 27 Dexterity saving throw or be under an effect identical to the Prismatic spray spell, but all damage done by the spell is increased to 97 (15d12) with half damage on a failed saving throw. 

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

Cast Spell (Costs 1-2 Actions). Cirathium casts a spell from his wizard spellcasting or sorcerer spellcasting. If this spell is of 6th level or lower then this uses 1 legendary action, if it is of 7th level or higher then 2 legendary actions are required. 

Dimensional Roar  (Costs 2 Actions). Cirathium lets out a thunderous roar that shakes the space and can be heard up to 3 miles away. Each creature within 500ft of Cirathium must make a DC 27 Wisdom saving throw or be deafened for 1 hour and be either pulled 90ft towards Cirathium or pushed 90ft away. A creature can reroll their saving throw at the end of their turn to rid themselves of the deafness. 

Crush (Costs 1 Actions). Cirathium jumps up to 30ft in any direction, the space they land on if it has any creatures those creatures must make a DC 27 Dexterity saving throw or take 28 (8d6) bludgeoning damage and be knocked prone and restrained. At the start of each turn those who failed the saving throw can remake it to break the restraint and move adjacent. On a successful saving throw a creature moves adjacent to Cirathium's new space and are not restrained or knocked prone. 

Prismatic Sphere (Costs 3 Actions). Cirathium casts a spell from their innate spellcasting.