Arturius Grymm, The Mad God

Awakened Mind. Arturius can telepathically communicate with a number of creatures equal to his charisma modifier and can reach their minds regardless of what plane they are on. The creatures can choose to make a wisdom saving throw against his spell save DC to attempt to block him out. On a success, the creatures are immune to this effect for 1d4 days. 

Thought Shield. Arturius's mind cannot be read by any means unless he allows it. Any psychic damage dealt to him is immediately dealt back to the original source in full. 

Legendary Resistance (4/day). Whenever Arturius fails a saving throw, he can choose to succeed instead. 

Spellcasting. Arturius is a 20th-level spellcaster. His spellcasting ability is Charisma (spell save DC 27, +17 to hit with spell attacks). Arturius has the following Warlock spells prepared:
Cantrips (at will): eldritch blast, chill touch, friends, mind sliver
1st level (4 slots): armor of agathys, arms of hadar, cause fear, witch bolt

2nd level (4 slots): crown of madness, enthrall, mirror image, suggestion

3rd level (4 slots): hunger of hadar, hypnotic pattern, major image

4th level (4 slots): hallucinatory terrain, sickening radiance

5th level (4 slots): dream, mislead

6th level (1 slot): mental prison*

7th level (1 slot): crown of stars*

8th level (1 slot): maddening darkness*

9th level (1 slot): psychic scream*

*these are mystic arcanum spells that Arturius can cast only once per day. Arturius follows the rules of pact magic.

Multiattack. Arturius can make 3 attacks on his turn, two of which can be spell attacks.

Psionic Blade. Melee Attack: +16 to hit, reach 5 ft, deals 23 (2d8 + 7) psychic damage on a successful hit. 

Entropic Ward. Arturius can magically ward himself against an attack and turn an enemy's failed strike into good luck for himself. When a creature makes an attack roll against him, he can use his reaction to impose disadvantage on that roll. If the attack misses him, his next attack roll against the creature has advantage if he makes it before the end of his next turn.

Create Thralls (Costs 1 Action).  Arturius can infect a number of incapacitated humanoid minds with his alien magic. The number of incapacitated humanoids is equal to his charisma modifier. Those creatures are then charmed by him until they are killed.

Maddening Gaze (Costs 2 Actions). Arturius can cause madness in the minds of creatures simply by looking at them. Any creature within 60 ft of him takes 6d12 psychic damage and must make a wisdom saving throw. On a failed save, the creature takes 2 levels of madness and is incapacitated until the remove curse spell is cast on them. On a successful save, the creature only takes half damage and is not driven mad. If the effect drops the creature to 0 hit points, they become a thrall under his influence. 

Wrath of the Mad God (Costs 0 Actions). When Arturius is dropped to 0 hp, the mortal shell that contains his eldritch form is broken, releasing the true god from within. When in his true form, his spell attack modifier and save dc both increase by +3, he becomes immune to all damage, his size changes to gargantuan, and he regains all of his expended spell slots and mystic arcanum spells. 

There are four large eyes on his body that, if hit directly, can damage Arturius while he is in this state. Each eye has an AC of 17 and 30 (1d20 + Arturius's constitution modifier) hit points. If all of the eyes are destroyed, then Arturius is also destroyed.