Ce'ngy'at (Mind Flayer Warlock)

Agonizing Blast. Ce'ngy'at deals an additional +6 damage (Charisma bonus) to the damage he deals on a hit from his eldritch blast cantrip.

Eldritch Master (1/long rest). Ce'ngy'at can spend 1 minute entreating his patron, Selhocazid, for aid to regain all his expended spell slots.

Eyes of the Rune Keeper. Ce'ngy'at can read all writing.

Witch Sight. Ce'ngy'at can see the true form of any shapechanger or creature concealed by illusion or transmutation magic while the creature is within 30 feet of him and within line of sight.

Magic Resistance. Ce'ngy'at has advantage on saving throws against spells and other magical effects.

Thought Shield. Ce'ngy'at's thoughts cannot be read by telepathy or other means unless he allows it. Also, whenever a creature deals him psychic damage, that creature takes the same amount of damage that he does.

Innate Spellcasting (Psionics). The mind flayer's innate spellcasting ability for psionics is Intelligence (spell save DC 17). He can innately cast the following spells, requiring no components:

At will: detect thoughts, levitate 
1/day: dominate monster, plane shift (self only)

Innate Spellcasting (Invocations & Mystic Arcanum). The warlock’s innate spellcasting ability for his invocations and Mystic Arcanum is Charisma. He can innately cast the following spells (spell save DC 21), requiring no material components::

At will: alter self, arcane eye, detect magic
1/day: etherealness, eyebite, feeblemind, power word kill

Spellcasting. Ce'ngy'at is a 20th-level spellcaster. His spellcasting ability is Charisma (spell save DC 21, +13 to hit with spell attacks). He regains his expended spell slots when he finishes a short or long rest. He knows the following warlock spells: 

Cantrips (at will): eldritch blast, mage hand, minor illusion, true strike
1st–5th level (4 5th-level slots): banishment, clairvoyance, dimension door, dispel magic, dominate person, dream, evard's black tentacles, fly, hallucinatory terrain, hold monster, phantasmal force,  scrying, sending, telekinesis, tongues

Spellcasting (Book of Shadows). Ce'ngy'at's Book of Shadows allows him to cast a number of warlock cantrips and rituals while the book is on his person. His spellcasting ability is Charisma (spell save DC 21, +13 to hit with spell attacks) for these spells. His Book of Shadows grant him the following spells: 

Cantrips (at will): ray of frost, sacred flame, shocking grasp
Rituals (+10 minutes to cast, no spell slot used: alarm, contact other plane, forbiddance, identify

Tentacles. Melee Weapon Attack: +9 to hit, reach 5 ft., one creature. Hit: 16 (2d10 + 5) psychic damage. If the creature is Medium or smaller, it is grappled (escape DC 17) and must succeed on a DC 17 Intelligence saving throw or be stunned until this grapple ends.

Extract Brain. Melee Weapon Attack: +9 to hit, reach 5 ft., one incapacitated humanoid grappled by the mind flayer. Hit: The target takes 55 (10d10) piercing damage. If this damage reduces the target to 0 hit points, the mind flayer kills the target by extracting and devouring its brain.

Mind Blast (Recharge 5–6). The mind flayer magically emits psychic energy in a 60-foot cone. Each creature in that area must succeed on a DC 17 Intelligence saving throw or take 23 (4d8 + 5) psychic damage and be stunned 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.

Regain Spell Slot (Rod of the Pact Keeper) (1/long rest). As an action, Ce'ngy'at can use his Rod of the Pact Keeper to regain one expended warlock spell slot once per long rest.

Create Thrall. Ce'ngy'at can use his action to touch an incapacitated humanoid and infects its mind with the alien magic of his patron, Selhocazid. The humanoid is then charmed by the warlock until a remove curse spell is cast on it, the charmed condition is removed from it, or Ce'ngy'at uses this feature again. Ce'ngy'at can communicate telepathically with the charmed creature as long as he is on the same plane of existence as it.

Gaze of Two Minds. Ce'ngy'at can use his action to touch a willing humanoid and perceive through its senses until the end of his next turn. As long as the creature is on the same plane of existence as the warlock, Ce'ngy'at can use his action on subsequent turns to maintain this connection, extending the duration until the end of his next turn. While perceiving through the other creature's senses, the warlock benefits from any special senses possessed by that creature, but is blinded and deafened to his own surroundings.

Horror of That Which Should Not Be (1/short rest). The warlock's patron, Selhocazid, bestows upon the mind flayer the ability to project the horrific and fearsome presence of the great old ones. As an action, Ce'ngy'at can cause each creature in a 10-foot cube originating from him to make a Charisma saving throw (DC 21). On a failed save, the creature is horrified by vivid hallucinations and is frightened until the end of the mind flayer's next turn.

Entropic Ward (1/short rest). When a creature makes an attack roll against Ce'ngy'at, he can use his reaction to impose disadvantage on that roll. If the attack misses as a result, his next attack against the creature has advantage if he makes it before the end of his next turn.