Aboleth
Large Aberration, lawful evil
5,900 XP
Special Abilities
Amphibious
The aboleth can breathe air and water.
Mucous Cloud
While underwater, the aboleth is surrounded by transformative mucus. A creature that touches the aboleth or that hits it with a melee attack while within 5 ft. 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 aboleth, the aboleth learns the creature's greatest desires if the aboleth can see the creature.
Actions
Multiattack
The aboleth makes three tentacle attacks.
Tentacle
Melee Weapon Attack: +9 to hit, reach 10 ft., one target. Hit: 12 (2d6 + 5) 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) 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 aboleth targets one creature it can see within 30 ft. of it. The target must succeed on a DC 14 Wisdom saving throw or be magically charmed by the aboleth until the aboleth dies or until it is on a different plane of existence from the target. The charmed target is under the aboleth's control and can't take reactions, and the aboleth 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 aboleth.
Legendary Actions
Detect
The aboleth makes a Wisdom (Perception) check.
Tail Swipe
The aboleth makes one tail attack.
Psychic Drain (Costs 2 Actions)
One creature charmed by the aboleth takes 10 (3d6) psychic damage, and the aboleth regains hit points equal to the damage the creature takes.
Quick Reference
Running This Encounter
The aboleth should leverage its intelligence (17) to set ambushes and use its environment to its advantage, controlling the battlefield through water manipulation. Begin combat with Psychic Scream to frighten and disorient the party, then use its legendary actions to maintain pressure. Position it where it can retreat into deep water while using ranged psychic attacks. Remember that its mucous cloud is its greatest defensive tool—creatures starting their turns in it must succeed on a DC 14 CON save or be unable to breathe air.
Combat Tactics
The aboleth prioritizes using Psychic Scream on its first turn, then follows with tentacle attacks against the nearest threats while keeping distance via movement. It uses its three legendary actions each round to cast spells (often causing fear or charm effects), move away from melee threats, and attack with tentacles again. If reduced below 68 HP, it should attempt to flee into murky water or cast spell-like abilities defensively.
Environment & Setting
Aboleth encounters work best in underground lakes, sunken ruins, or extensive cave systems with deep water for retreat. The terrain should include pillars, crevasses, or multiple water levels to maximize the creature's mobility advantage.