Alien Mind. If a creature tries to read the aboleth’s thoughts or deals psychic damage to it, that creature must succeed on a DC 18 Intelligence saving throw or be stunned for 1 minute. The stunned creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.

Amphibious. The aboleth can breathe air and water.

Phantasmagoria. While underwater, the aboleth is surrounded by a cloud of sticky mucus. A hostile creature that starts its turn within 20 ft. of the aboleth rolls a d20, on a result of 1-10 the creature has Disadvantage on Intelligence and Wisdom saving throws. On a result of 11-20, the creature has Vulnerability to Psychic damage. Additionally, any creature that touches the aboleth or hits it with an attack while within 20 ft. of it must succeed on a DC 18 Strength saving throw or be restrained until the end of the aboleth’s next turn.

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

Multiattack. The aboleth makes one Tail and two Tentacle attacks.

Tentacle. Melee Weapon Attack: +13 to hit, reach 10 ft., one target. Hit: 12 (2d6 + 5) bludgeoning damage plus 3 (1d6) psychic damage.

Tail Strike. Melee Weapon Attack: +13 to hit, reach 10 ft., one target. Hit: 14 (2d8 + 5) bludgeoning damage.

Phantasmal Swipe. Ranged Spell Attack: +6 to hit, reach 120 ft., one target. Hit: 14 (4d6 + 5) psychic damage and the creature is blinded until the end of its next turn. This attack targets the creatures Intelligence score instead of their AC. 

Psychic Scream (1/day). The aboleth sends forth a devastating wave of painful psychic energy. All creatures within 30 ft. of the aboleth take 40 psychic damage, reduced by an amount equal to their Intelligence score.

Enslave (Recharge 4–6). The aboleth targets one creature it can see within 30 feet of itself. The target must succeed on a DC 18 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.

The charmed target can repeat the saving throw whenever it takes damage, ending the effect on itself on a success. No more than once every 24 hours, the target can repeat the saving throw when it is at least 1 mile away from the aboleth.

Painful Choice. Whenever the aboleth takes damage from a creature, it can use its reaction afflict the creature with a psychic affliction. If the creature deals damage to the aboleth while the affliction is active, they take 21 (6d6) psychic damage and then their affliction ends. Otherwise, at the opens of the afflicted creatures turn, they can end the affliction by making a DC 18 Intelligence saving throw. The affliction also ends if the aboleth uses this ability to afflict another creature.

Reactive Ward (1/day). Whenever the aboleth takes damage, it can use its reaction to create a field of defensive psychic energy. Doing so grants the aboleth 30 temporary hit points and, while the temporary hit points remain, a +5 bonus to its AC and resistance to all damage except psychic.

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.

Phantasmal Swpie. The aboleth makes one phantasmal swipe 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 dealt.