Amphibious. The Abolithid can breathe air and water.

Mucous Cloud. While underwater, the Abolithid is surrounded by transformative mucus. A creature that touches the Abolithid 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.

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

Innate Spellcasting (Psionics). The Abolithid’s innate spellcasting ability is Intelligence (spell save DC 18). It can innately cast the following spells, requiring no components: • At will: detect thoughts, levitate • 1/day each: dominate monster, plane shift (self only)

Multiattack. The Abolithid makes three tentacle attacks.

Tentacle. Melee Weapon Attack: +9 to hit, reach 10 ft., one target. Hit: 14 (2d8 + 5) bludgeoning damage. If the target is Medium or smaller, it is grappled, escape DC 15, and must succeed on a DC 15 Intelligence saving throw or be stunned until this grapple ends.

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.

Enslave (3/Day). The Abolithid 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 Abolithid until the Abolithid dies or until it is on a different plane of existence from the target. The charmed target is under the Abolithid's control and can't take reactions, and the Abolithid 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 Abolithid.

Extract Brain. Melee Weapon Attack: +8 to hit, Reach 10 ft., one incapacitated humanoid grappled by the Abolithid. Hit: 55 (10d10) piercing damage. This attack can only be used on a incapacitated humanoid grappled by the Abolithid. If this damage reduces the target to 0 hit points, the Abolithid kills the target by extracting and devouring its brain.

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

Break Concentration.The Abolithid targets one creature within 120 feet of it with which it has a psychic link.The Abolithid breaks the creature’s concentration on a spell it has cast. The creature also takes 2 (1d4) psychic damage per level of the spell.

Tail Swipe. The Abolithid makes one tail attack.

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