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 1 meter of it must make a Difficulty 14 Fortitude 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.
Spellcasting. The aboleth uses Psionics (Int) to cast spells (spell save Difficulty 16, +8 to hit with spell attacks). It has 10 mana, a mana limit of 2, regains all expended mana when it finishes a long rest and regains half its total mana when it finishes a short rest. It knows the following spells:
1 mana: agonizing rebuke, call to mind, confusion, ego whip, haze senses, meld mindcrystal, mind over body, mind spike, mind trap, psychic fortress, sending, sense minds, thought shield, wandering mind
Tentacle (3/turn). Melee Weapon Attack: +9 to hit, reach 2 m. Hit: 12 (2d6 + 5) bludgeoning damage. If the target is a creature, it must succeed on a Difficulty 14 Fortitude saving throw or become diseased. After 1 minute, the diseased creature’s skin becomes translucent and slimy, the creature can’t regain health unless it is underwater, and the disease can be removed only by a disease-curing spell cast at 3 or more mana. 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 (1/turn). Melee Weapon Attack: +9 to hit, reach 2 m. Hit: 15 (3d6 + 5) bludgeoning damage.
Enslave (3/day). The aboleth targets one creature it can see within 5 meters of it. The target must succeed on a Difficulty 14 Will 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 2 kilometers away from the aboleth.
Legendary Actions (3)
Only one legendary 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 aboleth makes a Perception check.
Psychic Drain. One creature charmed by the aboleth takes 10 (3d6) psychic damage, and the aboleth regains health equal to the damage the creature takes.
Tail Swipe. The aboleth makes one tail attack.
On initiative count 20 (losing initiative ties), the aboleth takes a lair action to cause one of the following effects; the same effect can’t be used two rounds in a row:
The aboleth casts phantasmal force (no components required) on any number of creatures it can see within 10 meters of it. While maintaining concentration on this effect, the aboleth can’t take other lair actions. If a target succeeds on the saving throw or if the effect ends for it, the target is immune to the aboleth’s phantasmal force lair action for the next 24 hours, although such a creature can choose to be affected.
Pools of water within 20 meters of the aboleth surge outward in a grasping tide. Any creature on the ground within 4 meters of such a pool must succeed on a Difficulty 14 Reflex saving throw or be pulled up to 4 meters into the water and knocked prone. The aboleth can’t use this lair action again until it has used a different one.
Water in the aboleth’s lair magically becomes a conduit for the creature’s rage. The aboleth can target any number of creatures it can see in such water within 20 meters of it. A target must succeed on a Difficulty 14 Will saving throw or take 7 (2d6) psychic damage. The aboleth can’t use this lair action again until it has used a different one.
The region containing the aboleth's lair is warped by the aboleth’s magic, which creates one or more of the following effects:
Underground surfaces within 2 kilometers of the aboleth’s lair are slimy and wet and are difficult terrain.
Water sources within 2 kilometers of the lair are supernaturally fouled. Enemies of the aboleth that drink such water vomit it within minutes.
As an action, the aboleth can create an illusory image of itself within 2 kilometers of the lair. The copy can appear at any location the aboleth has seen before or in any location a creature charmed by the aboleth can currently see. Once created, the image lasts for as long as the aboleth maintains concentration, as if concentrating on a spell. Although the image is intangible, it looks, sounds, and can move like the aboleth. The aboleth can sense, speak, and use telepathy from the image’s position as if present at that position. If the image takes any damage, it disappears.
If the aboleth dies, the first two effects fade over the course of 3d10 days.
No concoctions, maneuvers, or spells
All creatures in this system should have maneuvers or spells. You should add some maneuvers or spells to this creature. If you do so, please make a suggestion on github so I can finish adding maneuvers and spells to all creatures via https://github.com/mlenser/kryx-rpg-issues/issues/9.