Awareness.

The mind flayer has advantage on initiative rolls and can't be surprised as long as it doesn't have the incapacitated condition.

Magic Resistance.

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

Actions

Tentacles.

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

Extract Brain.

Melee Weapon Attack: +8 to hit, reach 5 ft., one Humanoid grappled by the mind flayer. Hit: 55 (10d10) piercing damage. If this damage reduces the target to 0 hit points, the mind flayer kills it by extracting and devouring its brain.

Mind Whip (Recharge 5–6).

The mind flayer lashes out with psychic energy, targeting up to two creatures it can see within 60 feet of itself. Each target must succeed on a DC 16 Intelligence saving throw or take 23 (4d8 + 5) psychic damage and have the stunned condition for 1 minute. A stunned target can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.

Spellcasting (Psionics).

The mind flayer casts one of the following spells, requiring no spell components and using Intelligence as the spellcasting ability (spell save DC 16):

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

Mind Flayer Prophet