Large Aberration, Lawful Neutral

Armor Class: 17 (Natural Armor)

Hit Points: 112 (15d10+30)

Speed: 25 ft. fly 35 ft. 

STR 18/+4

DEX 14/+2

CON 14/+2

INT 25/+7

WIS 8/-1

CHA 6/-2

Saving Throws: Con +6, Wis +3, Cha +2

Skills: Arcana +11, History +11, Insight +7, Nature +11, Perception +7, Religion +11

Damage Immunities: Psychic

Condition Immunities: Frightened, Charimed

Senses: Truesight 60 ft., Passive Perception 17

Languages: All

Challenge: 10

Proficiency Bonus: +4

Turn After Turn. A master of predicting and outmanoeuvring its opponents; each time an enemy character within 120 ft. of the Stockfish uses their Action, Stockfish can immediately move up to its speed and use any of its actions.

Mate in 549. Due to his enormous intelligence and tactical genius, Stockfish can often predict exactly what will happen in combat and adapts to the situation, especially if his enemies are predictable

• At the start of its turn, Stockfish becomes resistant to each type of damage and immune to each negative condition done to it during its previous turn. This effect lasts  until the start of its next turn. 
• Stockfish cannot be surprised.
• If an enemy’s action is exactly the same action as their last action (attack with the same weapon, same spell, etc…), the enemy takes 1d10 psychic damage, Stockfish gains 1d10 temporary Hit Points and an advantage on attack rolls and saving throws against that creature until the end of its next turn.

Engine Move. Each time Stockfish uses an action, there is a 1/20 chance that it will spend his action to do nothing, however it will become affected by the Foresight spell for 1 round after 1d4 rounds.

Battle-Tactics. Stockfish knows the following maneuvers: Disarming Attack, Grappling Attack, Lunging Attack, Precision Attack and Sweeping Attack, and can use one of them once per round. (1d8 extra die, DC 16).

Actions.

• Brute Force. Melee Weapon Attack: +8 to hit, reach 10 ft., one target. Hit: 13 (2d8+4) bludgeoning damage.
• Calculations. Stockfish makes a DC 15 Intelligence check, if it succeeds, it adds 1d4 to all of its rolls until the end of its next turn.
• Counterplay. Stockfish automatically succeeds on its next saving throw, or the next attack targeting it deals no damage. 
• Check. Ranged Spell Attack: +8 to hit, range 60 ft., one target. Hit: 8(1d1+7) psychic damage. Any creature hit by this attack must succeed on a DC 14 Charisma saving throw, becoming restrained until the end of their next turn on a failure.