Amphibious. The Aboleth Servitor Hulk  can breathe air and water.

Mucous Cloud. While underwater, the aboleth Servitor Hulk is surrounded by  mucus.

Rally. The Aboleth Servitor Hulk brings advantage on attack rolls to all allies within 10 squares of it.

Multiattack. The Aboleth Servitor Hulk makes three Raging Slam attacks.

Raging Slam. Melee Weapon Attack: +9 to hit, reach 10 ft., one target. Hit: 20 (2d8 + 4) 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)  psychic damage every 10 minutes unless moisture is applied to the skin before 10 minutes have passed.

The Aboleth Servitor Hulk 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 Servitor Hulk  regains spent legendary actions at the start of its turn.

Detect. The Aboleth Servitor Hulk  makes a Wisdom (Perception) check.

Raging Strike. The Aboleth Servitor Hulk  makes one raging slam attack.

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