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

Psionics - save DC 17. 
At will: Detect Thoughts,  Levitate
1/day each: Confusion, Dominate Monster, Eyebite, Feeblemind, Mass Suggestion

Unearthly Life Force. When Xor´chylic is reduced to 0 HP, they make 3 death saving throws at the start of each of their turns, but remains conscious. On a success, any creatures psychically connected to them take 1d6 psychic damage, and Xor´chylic heals half the damage dealt. On a failure, Xor´chylic´s brain begins consuming itself for psychic energy, and they take 1d6 Intelligence damage.

Tentacles. +8 to hit, 10 ft. reach, 4d10+5 psychic damage and grappled (DC 14 escape), DC 17 Int save or STUN until grapple ends

Extract Brain.+8 to hit, must be incapacitated and grappled, 10d10 piercing. If reduced to 0, instakill

Mind Blast (Recharge 5-6). 60 ft. cone, DC 16 Int save or take 4d12+5 psychic damage and STUN 1 minute. Save ends end of turn

Thought Scramble: Each creature that attempts to act before the mindflayer in the initiative order is forced to make an Intelligence saving throw or have their initiative order reset to a random value. The effect lasts until the end of the next round.

Hive Sacrifice: When the mindflayer is targeted by a spell or effect requiring a mental saving throw, the mindflayer can choose to have the spell target one of its thralls within 100 feet instead. The thrall dies from the strain.

Psychic Camouflage: When the mindflayer takes damage, it can use its reaction to momentarily disappear from the mind of the attacker, causing all other attacks that round to be at disadvantage.

Stillness of Mind: The mindflayer can use a reaction to end an effect that causes it to be stunned, charmed, or frightened.