Multiattack. The dragon makes one Rend attack and either (A) one Tentacles attack or (B) a use of Extract Brain.

Rend. Melee Attack Roll: +6, reach 10 ft. Hit: 12 (2d8 + 3) Slashing damage.

Tentacles. Melee Attack Roll: +5, reach 15 ft. Hit: 24 (4d10 + 2) Psychic damage, and if the target is Huge or smaller, it is Grappled (escape DC 14).

Extract Brain. Intelligence Saving Throw: DC 13, one creature within 30 feet of the dragon. Failure: 27 (5d10) Psychic damage, and until the target finishes a Long Rest or dies, its Proficiency Bonus is cumulatively lowered by 1. The dragon gains a cumulative +2 bonus to damage rolls. A creature whose Proficiency Bonus drops to 0 can’t form new thoughts or speak, and it has Disadvantage on attack rolls and d20 Tests. Success: Half damage, and the target's Proficiency Bonus is not reduced. Failure or Success: If this damage reduces the target to 0 Hit Points, the dragon kills it and devours its brain.

Mind Blast (Recharge 5-6). Intelligence Saving Throw: DC 13, each creature in a 30-foot cone. Failure: 38 (11d6) Psychic damage. Success: Half damage.

Weaken Will. Trigger: A creature Grappled by the dragon makes a saving throw against one of the dragon's abilities. Response: The creature has Disadvantage on the saving throw.