Magic Resistance. The mind flayer has Advantage on saving throws against spells and other magical effects.
Tentacles. Melee Attack Roll: +7, reach 5 ft. Hit: 22 (4d8 + 4) Psychic damage. If the target is a Medium or smaller creature, it has the Grappled condition (escape DC 14) from all the mind flayer's tentacles, and the target has the Stunned condition until the grapple ends.
Extract Brain. Constitution Saving Throw: DC 15, one creature that is Grappled by the mind flayer's Tentacles. Failure: 55 (10d10) Piercing damage. Success: Half damage. Failure or Success: If this damage reduces the target to 0 Hit Points, the mind flayer kills it and devours its brain.
Mind Blast (Recharge 5–6). Intelligence Saving Throw: DC 15, each creature in a 60-foot Cone. Failure: 31 (6d8 + 4) Psychic damage, and the target has the Stunned condition until the end of the mind flayer's next turn. Success: Half damage only.
Spellcasting. The mind flayer casts one of the following spells, requiring no spell components and using Intelligence as the spellcasting ability (spell save DC 15):
At will: Detect Thoughts