Chthulu of the ceaseless Dream

Thousand Mind: chthulhu can concentrate on up to 4 spells simultaneously, and has advantage on all concentration checks

Spellcasting. Chthulhu is a 20th-level spellcaster. Its spellcasting ability is charisma (spell save DC , 27 +19 to hit with spell attacks). Chthulhu knows all spells 
1st level (8 slots) 2nd level (8 slots) 3rd level ( 8 slots) 4th level (8 slots) 5th level (8 slots) 6th level (8 slots) 7th level(6 slots) 8th level (4 slots) 9th level (2 slots)

Multi attack. Cthulhu makes 4 attacks with the blade of worlds end, or 2 attacks and casts a spell

Blade of worlds end. Melee Weapon Attack: +20 to hit, reach 15 ft., single target. Hit: 44 (5d12 + 14) psychic damage. And the creature struck subtracks 1d6 from their next roll of any kind

Eldritch shower. RangedWeapon Attack: +24 to hit, range 120/300 ft., 1/5 target. Hit: 30 (2d10 + 10) force damage.

Thief of mind. If cthulhu sees a creature use a class feature he can use his reaction to gain a single use of that ability, this can be used 3 times and regains all uses after a long rest

 Worlds end (Costs 1 Actions). Cthulhu makes 1 attack with the blade of world end

chaotic spell (costs 2 actions). cthulhu casts a spell 

Mire mind (costs 1 action). Cthulhu selects a target eithin 120 feet and makes them use a badic attack against a target of his choice

Enter the description for how mythic actions work for your monster here.

Dream of endless torment (Costs 1Actions). Recharge 5/6 All creatures within 60ft of chthulu must make a wisdom saving throw vs his spell save dc. On a failure the creature enters a dream like state, suffering disadvantage on all attacks and saving throws, as well as tsking 2d12 damage every time they miss with an attack or fail a saving throw. A creature can use its action to retry the saving throw, waking themselves on a success, or a creature within 5ft can use its action to shake the effected creature awake, ending the effect in either case