Magic Immunity. Yimh'avhuth can't be affected or detected by spells. It has advantage on Saving Throws against magical effects.

Creature Sense. Yimh'avhuth the is aware of the presence of creatures within 5 miles of it that have an Intelligence score of 4 or higher. It knows the distance and direction to each creature, as well as each one's intelligence score, but can't sense anything else about it. A creature protected by a mind blank spell, a nondetection spell, or similar magic can't be perceived in this manner.

Maddening Aura. Creatures within 60 ft must pass an Intelligence saving throw DC 20 or take a permanent madness and become frightened. It may repeat the saving throw at the end of each of its turns. Targets that pass cannot be effect for 24 hours. Creatures with 20+ intelligence have advantage on the saving throw.

Psionic Strike. Yimh'avhuth's melee attacks use its Intelligence modifier for attack rolls and damage rolls, additionally its melee attacks are considered magical for the sake of overcoming immunity and resistances.

Spellcasting. Yimh'avhuth is a 20th-level spellcaster. Its spellcasting ability is Intelligence (spell save DC 27, +19 to hit with spell attacks). Yimh'avhuth has all spells know and all spells count as wizard spells to it. It can change the damage type and saving throw at will.

Cantrips (at will): All

1st level (4 slots): All

2nd level (3 slots): All

3rd level (3 slots): All

4th level (3 slots): All

5th level (3 slots): All

6th level (2 slots): All

7th level (2 slots): All

8th level (1 slot): All

9th level (1 slot): All

Multiattack.  Yimh'avhuth either 8 attacks with its Tentacle attack or substitute 2 Tentacle attacks with one Claw. 

Claw. Melee Weapon Attack: +19 to hit, reach 25 ft., 1 target. Hit: 31 (3d12 + 10) Slasing damage. 

Tentacle Melee Weapon Attack: +19 to hit, reach 15 ft., 1 target. Hit: 22 (3d6 + 10) bludgeoning damage. If the target is huge or smaller it is grappled. Yimh'avhuth may use its action to restrain any grappled targets. A target restrained by this takes 18 (2D6 + 10) bludgeoning damage and 18 (2D6 + 10) psychic damage. Until the grapple ends, Yimh'avhuth can attack with one less tentacle.

Perish Song. Upon hitting 0 hp, as a reaction Yimh'avhuth produces a psionic scream which resonates throughout the plane which it is in. All creatures within a 5 mile radius must make an Intelligence saving throw, taking 100 psychic damage on a fail and half as much on a success. Creatures with 20+ intelligence are immune to the affect, but still take the psychic damage. The closet creature to hit 0 hp from this scream downloads Yimh'avhuth's personality, which rewrites the original personality. The effect creatures is brought back to 1 hp and gains an intelligence score of 20

Yimh'avhuth can take 3 legendary actions, choosing from the options below. Only one legendary action option can be use at a time and only at the end of another creature's turn. Yimh'avhuth regains spent legendary actions at the start of its turn.

Attack Yimh'avhuth makes either one Claw or Tentacle attack.

Spell Yimh'avhuth casts a spell with a casting time of an action, bonus action, or reaction.

Movement Yimh'avhuth moves up to half its speed.

Gravity shift (Cost 2 Actions) Yimh'avhuth changes the terrain and gravity of the surrounding environment in a 100 ft radius. 

Teleport (Cost 2 Actions)  Yimh'avhuth teleports up to 120 ft.

Recharge (Cost 3 Actions)  Roll a D6 + 2, Yimh'avhuth regenerates a spell slot equal to the rolled number.