Watcher Of The Gods

Amphibious. The creature can breathe air and water.

Hidden in Water: When reduced to 0 HP from 1000 HP, the creature does not die but retreats for later encounters

Legendary Resistance (3/Day, or 4/Day in Lair). If the aboleth fails a saving throw, it can choose to succeed instead.

Necrotic Resistance: Resistant to Necrotic Damage

Probing Telepathy. Creature can talk to target(s) around 100 feet and communicates telepathically

Multiattack. The Watcher makes two Tentacle attacks and uses Eat The Soul

Tentacle. Melee Attack Roll: +10 to hit, reach 60 ft. One target. Hit: 8d10 + 9. 

Tentacle Slam: Attacks with Tentacle twice in one turn. +10 to hit, reach 60 ft

Eat The Soul: Creatures affected by Stun status or Downed status are reduced to 0 Health or killed. Watcher also has the Engorged status. Which means it heals for 4d10 Hit Points and gains 4d10 temporary health. Creatures killed by Eat The Soul cannot be revived until their soul is taken back to their body, which is only achieved through disrupting the concentration. Concentration DC is 10 or half of the damage taken to succeed.

Dominate Mind (2/Day). Wisdom Saving Throw: DC 16, one creature the aboleth can see within 30 feet. Failure: The target has the Charmed condition until the Watcher dies or is on a different plane of existence from the target. While Charmed, the target acts as an ally to the Watcher and is under its control while within 60 feet of it. In addition, the Watcher and the target can communicate telepathically with each other over any distance.

The target repeats the save whenever it takes damage as well as after every 24 hours it spends at least 1 mile away from the aboleth, ending the effect on itself on a success.

Tentacle Sweep: +10 to hit, 60 feet. Can hit up to 3 targets if they're close to each other. Hit: 8d10 + 10

Feast Of The Ages: When Engorged can heal for 3d10 Hit Points every turn as a bonus action

Halting Screech:  After being attacked, as a reaction, the Watcher emits a powerful screech ranging 30 ft, that deals 10d10 Psychic damage. Wisdom Saving Throw: DC 16,. Failure: the target(s) have the Frightened condition until attacked or after 10 turns. Success: Take 10d10 damage 

Psychic Rebuke: After being attacked, the creature can use a reaction to deal 5d10 damage on the target. 60 feet. Failed: 5d10 damage, Success: Take half damage.

Tentacle Slam: After taking damage, it can choose to deal 6d10 damage on the target, with a range of 30 feet.

Legendary Action Uses: 3 (4 in Lair). Immediately after another creature’s turn, the aboleth can expend a use to take one of the following actions. The aboleth regains all expended uses at the start of each of its turns.

Water War Missiles: Rain down missiles made from water that deal 5d10 damage to each member in a 30-foot radius, in a circle

Fear The Deep: Creature casts Frightened on a nearby creature, that creature must make a DC 15 Con save not to get Frightened

Slam: Nearby slam makes creatures in a 60 ft radius to take 5d10 Force damage