Ancient Brainstealer Dragon

Legendary Resistance (3/Day). If the dragon fails a saving throw, it can choose to succeed instead.

Innate Spellcasting (Psionics). The dragon’s innate spellcasting ability is Intelligence (spell save DC 19). It can innately cast the following spells, requiring no components:

At will: detect thoughts, levitate

1/day each: dominate monster, plane shift (self only)

Multiattack. The dragon can use its Frightful Presence. It then makes three attacks: one with its tentacle attack and two with its claws.

Tentacle. Melee Weapon Attack: +15 to hit, reach 15 ft., one target. Hit: 19 (2d8 + 8) bludgeoning damage plus 9 (2d8) acid damage. If the target is large or smaller it is grappled (escape DC 23), it must make a DC19 Intelligence saving throw or be stunned until this grapple ends. 

Claw. Melee Weapon Attack: +15 to hit, reach 10 ft., one target. Hit: 15 (2d6 + 8) slashing damage.

Tail. Melee Weapon Attack: +15 to hit, reach 20 ft., one target. Hit: 17 (2d8 + 8) bludgeoning damage.

Frightful Presence. Each creature of the dragon's choice that is within 120 feet of the dragon and aware of it must succeed on a DC 19 Wisdom saving throw or become frightened 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. If a creature's saving throw is successful or the effect ends for it, the creature is immune to the dragon's Frightful Presence for the next 24 hours.

Extract Brain. Melee Weapon Attack: +15 to hit, reach 5 ft., one incapacitated humanoid grappled by the dragon. Hit: The target takes 83 (15d10) 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 5–6). The dragon lets out a blast of psychic energy in a 90-foot cone. Each creature in the cone must make a DC 19 Intelligence saving throw, taking 45 (10d8) psychic damage and becoming stunned for one minute on a failed save. On a successful save, effected creatures take half as much damage and are not stunned. Stunned creatures can repeat this saving throw at the end of each of their turns, ending the effect on themselves on a success. 

The dragon can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature's turn. The dragon regains spent legendary actions at the start of its turn.

Detect Sentience. The Dragon detects the presence and location of any creature within 120 feet of it that has an Intelligence of 3 or higher, regardless of interposing barriers, unless the creature is protected by a mind blank spell.

Tail Attack. The dragon makes a tail attack.

Telekinetic Repulsion(Costs 2 Actions). The dragon lets out a blast of telekinetic energy. Each creature within 30 feet of the dragon must succeed on a DC 19 Strength saving throw or take 15 (2d6 + 8) bludgeoning damage and be knocked 10 feet away from the dragon and prone.