Amphibious. The aboleth can breathe air and water.

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

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 must make a DC 17 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.

Innate Spellcasting (Psionics). The aboleth's innate spellcasting ability is Intelligence (spell save DC 20). The aboleth can innately cast the following spells, without providing material components:

At will: hypnotic pattern, invisibility, phantasmal force

3/day each: hallucinatory terrain, major image

2/day each: phantasmal killer, project image

1/day each: mirage arcane, psychic scream, weird, plane shift(self only)

Multiattack. The aboleth makes four tentacle attacks.

Tentacle. Melee Weapon Attack: +14 to hit, reach 10 ft., one target. Hit: 18 (2d10 + 7) bludgeoning damage and the target is grappled (escape DC 18). If the target is a creature, it must succeed on a DC 19 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: +14 to hit, reach 10 ft., one target. Hit: 23 (3d10 + 7) bludgeoning damage.

Enslave (3/Day). The aboleth targets one creature it can see within 30 feet of it. The target must succeed on a DC 19 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.

Fling. One Large or smaller object held or creature grappled by the aboleth is thrown up to 60 feet and knocked prone. If a thrown target strikes a solid surface, the target takes 3 (1d6) bludgeoning damage for every 10 feet it was thrown. If the target is thrown at another creature, that creature must succeed on a DC 18 Dexterity saving throw or take the same damage and be knocked prone. If the aboleth uses multiattack, it can use its Fling in place of one of its tentacle attacks.

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.

Enslave. The aboleth uses its Enslave feature.

Tail Swipe. The aboleth makes one tail attack.

Psychic Drain (Costs 2 Actions). One creature charmed by the aboleth takes 28 (8d6) psychic damage, and the aboleth regains hit points equal to the damage the creature takes.

Cast a Spell (Costs 3 Actions). The aboleth casts a spell from its list of innate spells, using a spell slot as normal.