Corrupted Mind Flayer

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

Mage Hand Trickery.  The corrupted mind flayer can cast mage hand as a bonus action, and can use this to distract an enemy target to gain advantage on a Tentacle Tail attack.

Innate Spellcasting (Psionics). The corrupted mind flayer’s innate spellcasting ability is Intelligence (spell save DC 15, +5 to attack rolls). Because its spellcasting is so alien to any other arcane magic user, their spells cannot be countered.  The corrupted mind flayer can innately cast the following spells, requiring no components:

• At will: minor illusion, levitate, mage hand, sacred flame (psychic instead of radiant damage)
• 1/day each: Armor of Agathys
• 2/day each: shield (self only), heat metal

The Corrupted Mind Flayer can make one Tentacle Tail attack, and one cantrip spell cast, per round, or perform one Mind Blast, or it can cast one of its other spells.

Tentacle Tail. Melee Weapon Attack: +7 to hit, reach 5 ft., one creature. Hit: 15 (1d10 + 2) psychic damage. If the target is Medium or smaller, it is grappled (escape DC 16) and must succeed on a DC 14 Intelligence saving throw or be stunned until this grapple ends.

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 15 Intelligence saving throw or take 16 (3d8 + 2) 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.