The Rock

 Reflective Form: Any time the Rock is targeted by a magic missile spell, a line spell, or a spell that requires a ranged attack roll, roll a d6. On a 1 -3, the Rock is Affected. On a 4-5 the Rock is Unaffected. On a 6, the Rock is unaffected, and the effect is reflected back at the caster as though it originated from the Rock, turning the caster into the target.

False Appearance. While the Rock remains motionless, it is indistinguishable from an ordinary boulder.

Immutable Form. The golem is immune to any spell or effect that would alter its form.

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

Ensnaring Body. The Rock is a selfish being, and its body understands this. Any weapon attack that meets without exceeding the AC of the rock gets stuck in its body. A creature can attempt a DC 16 athletics check to dislodge the weapon when it becomes lodged and afterward as an action.

Legendary Resistances 3/day. When the Rock fails a saving throw, it may instead choose to succeed.

Multiattack. The Rock makes two Psychic Lash attacks.

Psychic Lash. Melee/Ranged Weapon Attack: +8 to hit, reach 5/60ft., 1 target. Hit: 19 (2d12 + 5) psychic damage

Psychic Pulse (Recharge 5–6). The mind flayer magically emits psychic energy in a 60-foot sphere. Each creature in that area must succeed on a DC 15 Intelligence saving throw or take 22 (4d8 + 4) psychic damage and gain 1 level of Exhaustion. 

The Rock has three legendary actions, which it can use at the end of another creature's turn and recharge at the beginning of its own turn.

Can You Smell (Costs 1 Action). The Rock begins emitting a noxious order from it's body; every creature within 30ft. must succeed on a DC 17 Constitution saving throw or become poisoned for 1 minute.

Teleport (Costs 2 Actions). The Rock has a short-range teleport up to 60ft. that the Rock can see.

Psychic Storm. (Costs 3 Actions). The Rock slightly opens up and shoots out the magical bursts of psychic energy that come raining down around. Every creature within 50 ft. of the rock must make a DC 17 Dexterity Saving throw. On a failed save, the creature takes 15 (5d6) Psychic Damage and 15(5d6) Force Damage or half as much on a successful one.