Amphibious. The YggLong can breathe air and water.

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

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

Multiattack. The YggLong makes two attacks.

Constriction. Melee Weapon Attack: +6 to hit, reach 10 ft., one target. Hit: 12 (2d6 + 5) bludgeoning damage.  (1d12) 

Tail. Melee Weapon Attack: +6 to hit, reach 10 ft. one target. Hit: 15 (3d6 + 5) bludgeoning damage.

The YggLong 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.

Detect. The YggLong makes a Wisdom (Perception) check.

Tail Swipe. The YggLong makes one tail attack.

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