Magic Resistance. The mind flayer has advantage on saving throws against spells and other magical effects.

Arcane Recovery. Once per day when the mind flayer finishes a short rest, it can choose expended spell slots to recover. The spell slots can have a combined level that is equal to or less than 5.

Portent. When the mind flayer finished a long rest, roll two d20s and record the numbers rolled. You can replace any attack roll, saving throw, or ability check made by the mind flayer or a creature that it can see with one of these foretelling rolls. You must choose to do so before the roll, and you can replace a roll in this way only once per turn
Each foretelling roll can be used only once. When the mind flayer finished a long rest, you lose any unused foretelling rolls.

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

At will: detect thoughts, levitate

1/day each: dominate monster, plane shift (self only)

Spellcasting. The mind flayer is a 10th-level spellcaster. Its spellcasting ability is intelligence (spell save DC 16, +8 to hit with spell attacks). Any of these spells with an (R) after it can be cast as a ritual without expending a spell slot. The mind flayer has the following wizard spells prepared:

Cantrips (at will): dancing lights, mage hand, minor illusion, shocking grasp

1st level(4 slots): detect magic(R), disguise self, shield, sleep

2nd level(3 slots): blur, invisibility, ray of enfeeblement

3rd level(3 slots): clairvoyance, lightning bolt, sending

4th level(3 slots): confusion, hallucinatory terrain

5th level (2 slots): telekinesis, wall of force

Spellbook. The mind flayer has the following wizard spells in its book and can swap these for any prepared spell after a long rest.  Any of these spells with an (R) after it can be cast as a ritual.

1st level: comprehend languages(R), find familiar(R), identify(R), illusory script(R), floating disk(R), unseen servant(R)

2nd level: gentle repose(R), magic mouth(R), skywrite(R)

3rd level: tiny hut(R), water breathing(R)

4th level: (all currently prepared)

5th level:  contact other plane(R), telepathic bond(R)

The Third Eye. The mind flayer gives itself one of the following benefits, which lasts until it is incapacitated or it takes a short or long rest. The mind flayer can't use the feature again until it finishes a short or long rest.

Ethereal Sight. The mind flayer can see into the Ethereal Plane within 60 feet of it.

Greater Comprehension. The mind flayer can read any language.

See Invisibility. The mind flayer can see invisible creatures and objects within 10 feet of it that are within line of sight.

Tentacles. Melee Weapon Attack: + 8 to hit, reach 5 ft., 1 target. Hit: 15 (2d10 + 4) psychic damage. If the target is Medium or smaller, it is grappled (escape DC 16) and must succeed on a DC 16 Intelligence saving throw or be stunned until this grapple ends.

Extract Brain. Melee Weapon Attack: + 8 to hit, reach 5 ft., 1 target that is incapacitaed and grappled by the mind flayer. Hit: 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 16 Intelligence saving throw or take 22 (4d8 + 4) 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.