The aboleth can breathe air and water.
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 14 Constitution saving throw. On a failure, the creature is diseased for 1d4 hours. The diseased creature can breathe only underwater.
If a creature communicates telepathically with the aboleth, the aboleth learns the creature’s greatest desires if the aboleth can see the creature.
The aboleth makes three tentacle attacks.
Tentacle.Melee Weapon Attack: +9 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 14 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: +9 to hit, reach 10 ft. one target. Hit: 15 (3d6 + 5) bludgeoning damage.
The aboleth 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 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 creature 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 creature regains spent legendary actions at the start of its turn.
The aboleth makes a Wisdom (Perception) check.
The aboleth makes one tail attack.
Psychic Drain (Costs 2 Actions).
One creature charmed by the aboleth takes 10 (3d6) psychic damage, and the aboleth regains hit points equal to the damage the creature takes.
Source:Pathfinder Roleplaying Game Bestiary.
As befits their hideous primeval appearance, the aboleths are among the world’s oldest forms of life. Ancient even when the gods first turned their eyes to the Material Plane, the aboleths have always existed apart from other mortal life, alien and cold and endlessly plotting. They once ruled the world with vast empires, and today view most other forms of life as either food or slaves—and sometimes both. They disdain the gods and see themselves as the true masters of creation. An aboleth is 25 feet long and weighs 6,500 pounds.
In the darkest reaches of the sea, aboleths still dwell in grotesque cities built in nauseating and cyclopean styles. There they are served by countless slaves culled from every nation, air-breathing and aquatic alike, although the air-breathing slaves are doubly bound by magic and the need to constantly replenish their water-breathing ability via the excretions of their aboleth masters. Lone aboleths are often advance scouts for these hidden cities, seeking out new slaves.