Ashiok

Mind of Madness. Ashiok is immune to the charmed and frightened conditions. In addition, whenever a creature attempts to peer into Ashiok's thoughts via any means, it must make a Wisdom saving throw (DC 18). On a fail, it takes 2d6 psychic damage and its mind gets swarmed with nightmarish visions, causing it to behave erratically for 1 minute, as if under the effect of the confusion spell. The creature can repeat the saving throw at the end of each of its turns, ending the effect on a success.

Shadow Stealth. While in dim light or darkness, Ashiok can take the Hide action as a bonus action.

Sunlight Weakness. While in sunlight, Ashiok has disadvantage on attack rolls, ability checks, and saving throws.

Innate Spellcasting. Ashiok's innate spellcasting ability is Charisma (spell save DC 18, +10 to hit with spell attacks). Ashiok can innately cast the following spells, requiring no material components:
1/day each: plane shift

Spellcasting. Ashiok is an 18th-level spellcaster. Ashiok's spellcasting ability is Charisma (spell save DC 18, +10 to hit with spell attacks). Ashiok has the following spells prepared:
Cantrips (at will): message, minor illusion, toll the dead, thaumaturgy, vicious mockery 
1st level (4 slots): arms of Hadar, disguise self, dissonant whispers, mage armor
2nd level (3 slots): phantasmal force, silence, suggestion
3rd level (3 slots): fear, enemies abound, counterspell
4th level (3 slots): confusion, greater invisibility, shadow of moil
5th level (3 slots): dream, synaptic static, dominate person
6th level (1 slot): mental prison
7th level (1 slot): etherealness
8th level (1 slot): maddening darkness
9th level (1 slot): weird

Multiattack. Ashiok makes 2 Nightmare Pull attacks. 

Nightmare Pull. Melee Touch Attack: +10 to hit, reach 5 ft., 1 target. Hit: 19 (4d6 + 5) psychic damage. If the target is unconscious, it takes extra 10 (3d6) damage and is cursed for 1 minute. The mind of a creature cursed by Ashiok is beset by a manifestation of its worst fears. As long as the curse persists, the cursed creature is frightened, and must make a Wisdom saving throw (DC 18) at the end of each of its turns, taking 22 (4d10) psychic damage on a fail (as per the phantasmal killer spell). On a success, the curse ends. 

Compelled Slumber (Recharge 5-6). Ashiok's voice seeps into nearby creatures' thoughts, compelling them to fall asleep. Each creature within 60ft. of Ashiok must make a Wisdom saving throw (DC 18), or fall unconscious. A creature affected by this ability wakes up immediately after taking any damage. Alternatively, a creature unaffected by this ability can spend their action shaking an affected one awake. 

Dream Eater. When an unconscious creature Ashiok can see within 30 feet regains consciousness, Ashiok can force the creature to make a DC 18 Wisdom saving throw. Unless the save succeeds, the creature takes 11 (2d10) psychic damage, and Ashiok regains hit points equal to the amount of damage taken.

Ashiok 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. Ashiok regains spent legendary actions at the start of its turn.

Nightmare Pull (1 Action). Ashiok uses the Nightmare Pull attack.

Shadow Step (1 Action). Ashiok teleports to a space within 60ft. that is in dim light. This move does not provoke opportunity attacks. 

Cast Spell (2 Actions). Ashiok casts a spell of 3rd level or lower. Ashiok cannot use a spell slot higher than 3rd to cast this spell.