Multiattack

The aboleth attacks three times with its tentacle.

Tentacle

Melee Weapon Attack: +9 to hit, reach 15 ft., one target. Hit: 19 (4d6 + 5) bludgeoning damage. The aboleth can choose instead to deal 0 damage. If the target is a creature it makes a DC 16 Constitution saving throw. On a failure it contracts a disease called the Sea Change. On a success it is immune to this disease for 24 hours. While affected by this disease the target has disadvantage on Wisdom saving throws. After 1 hour the target grows gills it can breathe water its skin becomes slimy and it begins to suffocate if it goes 12 hours without being immersed in water for at least 1 hour. This disease can be removed with a disease-removing spell cast with at least a 4th-level spell slot and it ends 24 hours after the aboleth dies.

Slimy Cloud (1/Day, While Bloodied)

While underwater the aboleth exudes a cloud of inky slime in a 30-foot-radius sphere. Each non-aboleth creature in the area when the cloud appears makes a DC 16 Constitution saving throw. On a failure it takes 44 (8d10) poison damage and is poisoned for 1 minute. The slime extends around corners and the area is heavily obscured for 1 minute or until a strong current dissipates the cloud.