Amphibious. The aboleth swarm can breathe air and water.

Mucous Cloud. The aboleth swarm is surrounded by transformative mucus. A creature that touches any aboleths in the swarm or that hits any with a melee attack while within 5 feet of it must make a DC 20 Constitution saving throw. On a failure, the creature is diseased for 1d20 hours. The diseased creature can breathe only underwater.

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

Multiattack. The aboleth swarm makes nine tentacle attacks.

Tentacle. Melee Weapon Attack: +7 to hit, reach 10 ft., one target. Hit: 12 (2d6 + 5) bludgeoning damage. If the target is a creature, it must succeed on a DC 20 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 no longer be removed. 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: +9 to hit, reach 10 ft. one target. Hit: 15 (3d6 + 5) bludgeoning damage.

Enslave (3/Day). The aboleth swarm targets one creature it can see within 30 feet of it. The target must succeed on a DC 14 Wisdom saving throw or be magically charmed by the aboleth swarm until the aboleth swarm dies or until it is on a different plane of existence from the target. The charmed target is under the aboleth swarm’s control and can’t take reactions, and the aboleth swarm 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 swarm.

The aboleth swarm 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 swarm regains spent legendary actions at the start of its turn.

Detect. The aboleth swarm makes a Wisdom (Perception) check.

Tail Swipe. The aboleth swarm makes three tail attack.

Psychic Drain (Costs 2 Actions). One creature charmed by the aboleth swarm takes 10 (3d6) psychic damage, and the aboleth swarm regains hit points equal to the damage the creature takes.