Amphibious. The abolich can breathe air and water.

Mucous Cloud. While underwater, the abolich is surrounded by magical mucus. A creature that touches the abolich or that hits it with a melee attack while within 5 feet of it must make a DC 16 Constitution saving throw. On a failure, the creature is diseased for 1d6 hours. The diseased creature can breathe only underwater.

Probing Telepathy. If a creature communicates telepathically with the abolich, the abolich learns the creature's greatest desires if the abolich can see the creature.

Undead guard. (1/day). Once per day when hostile creatures are within 100 ft of the abolich, it can summon 1d6 ghouls and 2d4 shadows.

Spellcasting. The abolich is an 18th-level spellcaster. Its spellcasting ability is Intelligence (spell save DC 20, +12 to hit with spell attacks). The lich has the following wizard spells prepared:

Cantrips (at will): mage hand, prestidigitation, ray of frost
1st level (4 slots): detect magic, magic missile, shield, thunderwave
2nd level (3 slots): acid arrow, detect thoughts, invisibility, mirror image
3rd level (3 slots): animate dead, counterspell, dispel magic, fireball
4th level (3 slots): blight, dimension door
5th level (3 slots): cloudkill, scrying
6th level (1 slot): disintegrate, globe of invulnerability
7th level (1 slot): finger of death, plane shift
8th level (1 slot): dominate monster, power word stun
9th level (1 slot): power word kill

Turn Resistance. The abolich has advantage on saving throws against any effect that turns undead.

Multiattack. The abolich makes two tentacle attacks, and casts a cantrip.

Tentacle. Melee Weapon Attack: +10 to hit, reach 10 ft., one target. Hit: 6 (1d6 + 3) bludgeoning damage and 6 (1d6+3) necrotic damage. If the target is a creature, it must succeed on a DC 15 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 7 (1d12+1) necrotic damage every 10 minutes unless moisture is applied to the skin before 10 minutes have passed.

Tail. Melee Weapon Attack: +10 to hit, reach 10 ft. one target. Hit: 11 (2d6 + 5) bludgeoning damage, and 3(1d6) necrotic damage.

Enslave (3/Day). The abolich targets one creature it can see within 35 feet of it. The target must succeed on a DC 15 Wisdom saving throw or be magically charmed by the abolich until the abolich dies or until it is on a different plane of existence from the target. The charmed target is under the abolich's control, and the abolich 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 abolich.

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.

Cantrip. The abolich casts a cantrip.

Tail Swipe. The abolich makes one tail attack.

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