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

Magic Resistance. The adult brainstealer dragon has advantage on saving throws against spells and other magical effects.

Innate Spellcasting (Psionics). The adult brainstealer dragon's innate spellcasting ability is Intelligence (spell save DC 18). The adult brainstealer dragon can innately cast the following spells, requiring no material or semantic components:

At will: suggestion 

3/day each: levitate, charm monster

Multi attack. The adult brainstealer dragon uses its frightful presence and makes one tentacles attack and two claw attacks. 

Tentacles. Melee Weapon Attack: +10 to hit, reach 10 ft., 1 target. Hit: 21 (4d8 + 5) psychic damage. If the target is Large or smaller, it is grappled (escape DC 17) and must succeed on a DC 14 Intelligence saving throw or be stunned until this grapple ends.

Claw. Melee Weapon Attack: +12 to hit, reach 10 ft., 1 target. Hit: 15 (2d8 + 7) [damage type] damage. 

Tail. Melee Weapon Attack: +12 to hit, reach 15 ft., 1 target. Hit: 13 (2d6 + 7) 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 17 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: +12 to hit, reach 5 ft., one incapacitated humanoid grappled by the adult brainstealer dragon. Hit: The target takes 65 (13d10) piercing damage. If this damage reduces the target to 0 hit points, the adult Brainstealer dragon kills the target by extracting and devouring its brain.

Mind Blast (Recharge 5-6). The adult Brainstealer dragon exhales psychic energy in a 60-foot cone. Each creature in that area must succeed on a DC 17 Intelligence saving throw or take 21 (4d8 + 5) 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.

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 minds (Costs 1 Actions). The dragon makes a Wisdom (Perception) check and if they detect a creature they also know that creature's intelligence score. 

Tail attack (Costs 1 Actions). The dragon makes a tail attack. 

Wing attack (Costs 2 Actions). The dragon beats its wings. Each creature within 10 ft. of the dragon must succeed on a DC 17 Dexterity saving throw or take 13 (2d6 + 7) bludgeoning damage and be knocked prone. The dragon can then fly up to half its flying speed.