Aboleth

Aboleths are extremely powerful aberrations, usually residing deep in the Underdark. Next to nothing is known about them, despite the fact that they are incredibly ancient and long-lived. They are known to possess perfect racial memories, remembering everything that happens to them and passing those memories on to their offspring.

Abilities
The following is all taken from the Monster Manual, 5th Edition, as aboleths have only been encountered at a distance onstream.
 * Amphibious: The aboleth can breathe air and water.
 * Probing Telepathy: The aboleth learns the greatest desires of any creature it can see that communicates with it telepathically.
 * Enslave (3/day): As an action, forces target creature to make a Wisdom saving throw or become magically charmed and under the aboleth's control, unable to take reactions. The aboleth can telepathically communicate over any distance with the charmed creature until the creature is on a different plane. Whenever the creature takes damage, or once per day when more than a mile away from the aboleth, it can repeat the saving throw.
 * Mucous Cloud: While underwater, a creature within 5 feet that touches the aboleth or hits it with a melee attack must succeed on a Constitution saving throw or become diseased and can breathe only underwater.
 * Tentacle: A creature hit by the tentacle attack must succeed on a Constitution saving throw or become diseased. After one minute, a diseased creature can't regain hit points except underwater, and takes damage every 10 minutes unless its skin is moistened.
 * Legendary Actions (3/round)
 * Make a Perception check
 * One tail attack
 * Psychic Drain (costs 2 actions): A creature charmed by the aboleth takes psychic damage, and the aboleth regains equivalent hit points.
 * Lair Actions
 * Cast Phantasmal Force on any number of creatures within 60 ft.
 * Nearby pools of water surge and pull creatures in, knocking them prone.
 * Psychic damage to all nearby creatures in the water the aboleth can see.
 * Regional Effects
 * Water within 1 mile becomes undrinkable, and underground surfaces are slimy, difficult terrain.
 * The aboleth can project an image of itself within 1 mile (if it has seen the location, or one of its enslaved creatures is there) as if concentrating on a spell.

History
Aboleths are theorized to have originated in the Far Realm. This theory is correct: millennia before 812 PD, aboleths and other aberrations arrived in Exandria through a portal deep beneath the surface of the Bladeshimmer Shoreline and took slaves as they built an empire in the Underdark. From their capital of Salar, the Unseeable City, they slowly expanded toward the surface, but their empire was unintentionally crushed when the final battle of the Calamity sent shockwaves through Exandria, collapsing much of the cavern network. The survivors of that collapse have slowly rebuilt in the centuries since, and long exposure to their psychic energies has created a residue of azuremite in the walls of what are now called the Crystalfen Caverns. The lord of the aboleth brood that still plots against the surface world is Overseer Durrom.

Vox Machina encountered their first aboleth after they were caught in a subterranean river system below Emon, which dumped them into Crystalfen Lake.

Later, during their travels through the Underdark beneath Kraghammer, Vox Machina came to another underground lake. Some of the enemies they lured into a chasm fell into that lake and were quickly pulled beneath the surface. The party was soon warned by their new ally, an illithid named Clarota, that an ancient evil lurked beneath the waters. When pressed for more information, he named the evil an aboleth, and said they would be much better off if they avoided it altogether.

In the Tal'Dorei Campaign Setting sourcebook, set in 812 PD, the adventure hook Mining the Past features a lone, dying aboleth in the azuremite mines of the Crystalfen Caverns who can give the adventurers visions of the expansive cavern system and the Ruins of Salar. Another adventure hook, This Cave was Made for Me, involves an aboleth in the caves behind Tormor Falls causing people to disappear from nearby villages and even from Syngorn.

Notable aboleths

 * Overseer Durrom, lord of the aboleth brood of Salar