Amphibious. The aboleth can breathe air and water. Acidic

Mucous Cloud. While underwater, the aboleth is surrounded by transformative mucus. A creature that touches the aboleth or that hits it with a melee attack while within 5 feet of it takes 7 (2d6) acid damage and must succeed on a DC 16 Constitution saving throw. On a failure, the creature is diseased for 1d4 hours. The diseased creature can breathe only underwater.

Probing Telepathy. If a creature communicates telepathically with the aboleth, the aboleth learns the creature's greatest desires if the aboleth can see the creature.

Multiattack. The aboleth makes three tentacle attacks. Alternatively, it can use two slime orb attacks.

Slime Orb. Ranged Spell Attack: +11 to hit, range 120 ft., one target. Hit: 14 (4d6) acid damage and the target must succeed on a DC 16 Strength saving throw or be restrained until the end of the aboleth's next turn.

Tentacle. Melee Weapon Attack: +11 to hit, reach 10 ft., one target. Hit: 13 (2d6 + 6) bludgeoning damage plus 10 (3d6) acid damage. If the target is a creature, it must succeed on a DC 16 Constitution saving throw or become diseased. The disease has no effect for 1 minute and can be removed by any magic that cures disease. After 1 minute, the diseased creature's skin becomes translucent and slimy, the creature can't regain hit points unless it is underwater, and the disease can be removed only by heal or another disease-curing spell of 6th level or higher. When the creature is outside a body of water, it takes 6 (1d12) acid damage every 10 minutes unless moisture is applied to the skin before 10 minutes have passed.

Tail. Melee Weapon Attack: +11 to hit, reach 10 ft., one target. Hit: 15 (3d6 + 5) bludgeoning damage plus 10 (3d6) acid damage.

Enslave (3/Day). The aboleth targets one creature it can see within 30 feet of it. The target must succeed on a DC 16 Wisdom saving throw or be magically charmed by the aboleth until the aboleth dies or until it is on a different plane of existence from the target. The charmed target is under the aboleth's control and can't take reactions, and the aboleth and the target can communicate telepathically with each other over any distance. Whenever the charmed target takes damage, the target can repeat the saving throw. On a success, the effect ends. No more than once every 24 hours, the target can also repeat the saving throw when it is at least 1 mile away from the aboleth.

The aboleth can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature's turn. The aboleth regains spent legendary actions at the start of its turn.

Tail Swipe. The aboleth makes one tail attack.

Psychic Drain (Costs 2 Actions). One creature charmed by the aboleth takes 17 (5d6) psychic damage, and the aboleth regains hit points equal to the damage the creature takes.

Slime Sphere (Costs 3 Actions). The aboleth conjures a ball of acidic slime that explodes at a point it can see within 120 feet of it. Each creature in a 20-foot-radius sphere centered on that point must succeed on a DC 16 Strength saving throw. On a failed save, the target takes 14 (4d6) acid damage and is restrained, or half as much damage and isn't restrained on a successful one. A restrained creature takes 10 (3d6) acid damage at the start of each of the Aboleth's turns. A restrained creature can use an action to perform a DC 16 Strength (Athletics) or Dexterity (Acrobatics) check, ending the effect on itself on a success.