Aboleth

AC 17     Initiative +7 (17)
HP 150 (20d10 + 40)
Speed 10 ft., Swim 40 ft.

Ability Scores
STR 21 (+5) Save +5    DEX 9 (−1) Save +3    CON 15 (+2) Save +6
INT 18 (+4) Save +8    WIS 15 (+2) Save +6    CHA 18 (+4) Save +4

Skills History +12, Perception +10
Senses Darkvision 120 ft.; Passive Perception 20
Languages Deep Speech; telepathy 120 ft.
CR 10 (XP 5,900, or 7,200 in lair; PB +4)

Traits
Amphibious. The aboleth can breathe air and water.
Eldritch Restoration. If destroyed, the aboleth gains a new body in 5d10 days, reviving with all its Hit Points in the Far Realm or another location chosen by the GM.
Legendary Resistance (3/Day, or 4/Day in Lair). If the aboleth fails a saving throw, it can choose to succeed instead.
Mucus Cloud. While underwater, the aboleth is surrounded by mucus. Constitution Saving Throw: DC 14, each creature in a 5-foot emanation originating from the aboleth at the end of the aboleth’s turn. Failure: The target is cursed. Until the curse ends, the target’s skin becomes slimy, the target can breathe air and water, and it can’t regain Hit Points unless it is underwater. While the cursed creature is outside a body of water, it takes 6 (1d12) acid damage at the end of every 10 minutes unless moisture is applied to its skin first.
Probing Telepathy. If a creature the aboleth can see communicates telepathically with it, the aboleth learns the creature’s greatest desires.

Actions
Multiattack. The aboleth makes two Tentacle attacks and uses either Consume Memories or Dominate Mind if available.
Tentacle. Melee Attack: +9, reach 15 ft. Hit: 12 (2d6 + 5) bludgeoning damage. If the target is a Large or smaller creature, it is Grappled (escape DC 14).
Consume Memories. Intelligence Saving Throw: DC 16, one creature within 30 feet that is Charmed or Grappled by the aboleth. Failure: 10 (3d6) psychic damage. Success: Half damage. On failure or success, if the target is a Humanoid and is reduced to 0 HP, the aboleth gains its memories.
Dominate Mind (2/Day). Wisdom Saving Throw: DC 16, one creature the aboleth can see within 30 feet. Failure: Charmed until the aboleth dies or is on a different plane. While Charmed, the creature is under the aboleth’s control and can communicate telepathically with it at any distance. The creature repeats the save when it takes damage and every 24 hours it is at least 1 mile away.

Legendary Actions
Legendary Action Uses: 3 (4 in Lair). The aboleth regains all expended uses at the start of its turn. It can take one of the following actions after another creature’s turn:
Lash. The aboleth makes one Tentacle attack.
Psychic Drain. If the aboleth has at least one creature Charmed or Grappled, it uses Consume Memories and regains 5 (1d10) Hit Points.