Assassinate. During its first turn, the Shadow Flayer has advantage on attack rolls against any creature that hasn't taken a turn. Any hit the Shadow Flayer scores against a surprised creature is a critical hit.

Evasion. If the Shadow Flayer is subjected to an effect that allows it to make a Dexterity saving throw to take only half damage, the Shadow Flayer instead takes no damage if it succeeds on the saving throw, and only half damage if it fails.

Sneak Attack. Once per turn, the Shadow Flayer deals an extra 14 (4d6) damage when it hits a target with a weapon attack and has advantage on the attack roll, or when the target is within 5 feet of an ally of the Shadow Flayer that isn't incapacitated and the Shadow Flayer doesn't have disadvantage on the attack roll.

Shadowcloak. A shadow flayer can draw on its own inner stores of quintessence to briefly become invisible. As a bonus action and taking 5 points of damage, a shadow flayer becomes invisible for 1 round. Unlike with the invisibility spell, shadowcloak doesn’t end when a shadow flayer attacks.

Tentacles. Melee Weapon Attack: +6 to hit, reach 5 ft., one creature. Hit: 15 (2d10 + 4) psychic damage. If the target is Medium or smaller, it is grappled (escape DC 14) and must succeed on a DC 14 Intelligence saving throw or be stunned until this grapple ends.

Extract Brain. Melee Weapon Attack: +6 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 6). The mind flayer emits psychic energy in a 60-foot cone. Each creature in that area must succeed on a DC 14 Intelligence saving throw or take 13 (2d8 + 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.

Spear. Melee or Ranged Weapon Attack: +5 to hit, reach 5 ft. or range 20/60 ft., one target. Hit: 6 (1d6 + 3) piercing damage, or 8 (1d8 + 3) piercing damage if used with two hands to make a melee attack.

Poisoned Shortsword. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 6 (1d6 + 3) piercing damage, and the target must make a DC 14 Constitution saving throw, taking 18 (5d6) poison damage on a failed save, or half as much damage on a successful one.