Multiattack: The Overseer can attack twice with his Zole Whip

Breathe Water: The Overseer can only breathe water, they cannot breathe air.

Savage:  If the Overseer hits both attacks with its Zole Whip, it can attack 1 more time with it.

Brutal: The Overseer scores a critical hit on a 19 or 20, not just a 20. If the Overseer scores a critical hit with his Zole Whip. the target automatically fails the saving throw, and the reduced strength is 1d6 instead of 1d4. If a critical hit is scored, any attacks on the target until the end of the overseers next turn have advantage.

Submit: If the Aboleth overseer is within 5ft of a target that is reduced to 0 hit points by its Zole Whip, It can attempt to try and make the target submit to it. This costs no action. The target drops to 1 hitpoint instead, and must succeed on DC 11 constitution savings throw or submit to the overseer. A target that has submitted must follow every action the overseer tells it to do, even if it is directly harmful to it. After 3 days, the target transforms into a Mind Slave, identical to one created by a Aboleths charm ability. The target is freed from this condition if the overseer is killed, or if it goes beyond 5 miles of the target.

Warp Reaction: Directly after an initiative roll, the overseer can swap it’s initiative with that of a Mind Slave, as long as the Overseer and the Mind Slave are participating in the same combat initiative. 

Zole Whip: Melee weapon attack: +6 to hit, reach 5ft., one target. Hit 8 (1d8+4) piercing damage. Target must succeed on a DC 11 constitution saving throw or have their strength score reduced by 1d4 for the next hour. This effect can stack, but a targets strength score cannot go below 8.

Intimidate- Before the Aboleth Overseer makes an attack with its Zole Whip, it can attempt to intimidate a creature. This intimidation occurs right before the Overseer attacks. The target of the intimidation must succeed on a DC 13 constitution savings throw, or be intimidated. Any time the overseer attacks a intimidated target with Zole Whip, the attack bonus goes up to +7, the damage goes up to 1d8+6, and the saving throw goes up to DC 13. After the overseer finishes his turn, the target is not intimidated any more.

Living Shield: If the Aboleth Overseer is targeted with a ranged weapon attack, it can choose to switch places with a Mind Slave, as long as the Overseer is adjacent to the Mind Slave. If the Overseer switches places, the designated attacks target the Mind Slave instead.