Blightspawn Aboleth

Corrupted Aura: The Blightspawn's corrupted presence extends beyond the poisoned water. Any creature that starts its turn within 15 feet of the Blightspawn must succeed on a DC 19 Constitution saving throw or take 14  (4d6) necrotic damage and become poisoned until the end of its next turn. Creatures already poisoned take an additional 14  (4d6) necrotic damage.

Infecting Telepathy: The Blightspawn’s telepathic communication spreads the Blightcurse Plague.

Blightcurse Plague: A creature that communicates telepathically with the Blightspawn or takes psychic damage from it must succeed on a DC 19 Wisdom saving throw or be infected with the curse. While cursed, the creature's hit point maximum is reduced by 15 each day. If this reduces the creature’s hit point maximum to 0, it dies and rises as a plague wight under the Blightspawn’s control. The curse can only be removed by a Remove Curse or Greater Restoration spell.

Multiattack: The Blightspawn makes three tentacle attacks.

Tentacle: Melee Weapon Attack: +10 to hit, reach 20 ft., one target. Hit: 20 (3d8 + 6) bludgeoning damage plus 10 (3d6) necrotic damage. If the target is a creature, it must succeed on a DC 19 Strength saving throw or be grappled (escape DC 19). While grappled, the target is restrained. The Blightspawn can grapple up to two creatures at a time.

Corrupted Spray (Recharge 5–6): The Blightspawn expels a stream of poisoned water in a 60-foot cone. Each creature in the area must make a DC 19 Dexterity saving throw, taking 45 (10d8) poison damage on a failed save, or half as much on a successful one. Creatures that fail the save are also blinded until the end of their next turn.

Tentacle Slam: The Blightspawn slams a creature it is grappling into a surface. The target must make a DC 19 Strength saving throw or take 28 (4d10 + 6) bludgeoning damage and be knocked prone. On a success, the target takes half damage and isn’t knocked prone.

Psychic Lash: The Blightspawn targets one creature it can see within 120 feet. The target must succeed on a DC 19 Intelligence saving throw or take 33 (6d10) psychic damage. On a failure, the target is also stunned until the end of its next turn.

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

Tail Swipe (Costs 1 Action): The Blightspawn lashes its tail. Each creature within 10 feet must make a DC 19 Dexterity saving throw or take 17 (3d8 + 6) bludgeoning damage and be knocked prone.

Dreadful Whispers (Costs 2 Actions): The Blightspawn invades the mind of a creature within 60 feet. The target must succeed on a DC 19 Wisdom saving throw or be frightened for 1 minute. The frightened creature can repeat the saving throw at the end of its turns, ending the effect on a success.

Drag to the Depths (Costs 2 Actions): The Blightspawn moves up to 20 feet and pulls one creature it is grappling with it. If the Blightspawn enters the water, the grappled creature must succeed on a DC 19 Strength (Athletics) or Dexterity (Acrobatics) check to avoid being dragged into the water.

Poisonous Surge (Costs 3 Actions): The Blightspawn causes the poisoned water to churn into a deadly whirlpool. Any creature currently in the water must succeed on a DC 20 Strength saving throw or take 22  (5d8) poison damage and be dragged 20 feet underwater by the swirling current. On a successful save, the creature takes half damage and is not pulled under.