Monster Of The Week - Aboleth

In the time before the great pantheon of the gods, these gigantic creatures lurked in the primordial oceans and the underground seas.  They reached out with their superior minds and enslaved the burgeoning lifeforms of the mortal realm, pushing dominion that made them like gods.  Until the true gods took notice, smashing the aboleths' empires and freeing their enslaved beings.  
  Such an insult has never been forgotten.

General Knowledge - Aboleths in 5th Edition

Eternal Memories

  Aboleths have ancient, flawless memories, and pass on their knowledge from generation to generation.  This quality keeps the insult of the gods fresh and alive, perfectly preserved in their minds.  They are also treasure troves of ancient lore.  These entities play the long game; calling eons of planning to bear, patient and intricate.  

Gods in the Lake

Ancient aquatic beings, Aboleths dwell in the deep recesses of underground lakes and rivers, and unknown depths of the grand oceans.  They often reside in the Material Plane, and the Plane Of Water, but can show up just about anywhere with an underwater abyss.  In these deep lairs and the lands that surround them, Aboleths are like gods, demanding worship and obedience from those that live nearby.  
  They also add the experiences and knowledge of all they consume to their eternal memory, creating a lust for knowledge with all they come in contact with.  An Aboleth wields its telepathic powers to read the minds of creatures and know their desires, and they use this power to easily gain the creature's loyalty.  While in its lair, an Aboleth can augment its telepathy, creating the illusions of such fulfilled desires for its loyal servants...but it is still just an illusion.

Enemies of the Gods

With flawless memories, and connections eons long, the Aboleths forever recall the slight of the gods.  Their fall from power is written in stark clarity, for Aboleths never truly die.  If an Aboleth's body is destroyed, its spirit returns to the Elemental Plane Of Water, where another body is coalesced in a month or two.  Ultimately, the Aboleths dream of one day overthrowing the gods and regaining control, and dominion, over the world.  
  They've had eons to plan, and they never forget.  Truly, they are a great danger to the cosmos.

Legacy Lore

Aboleths are a race of malevolent, eel-like gargantuan Aberrations with frightening psionic and psychic abilities.
Fish-like amphibians of immense size, often reaching 20 feet or more (6.1 meters) in length and many weighing up to 7,000 pounds.  Though, as an Aboleth's body can live indefinitely if not destroyed, there are some cited at 60 feet in length, and others weighing upwards of 12,000 pounds.  They resembled a bizarre eel, with long, tubular bodies, as well as a tail at one end and two fins near the head and another along the back. Aboleths' mouths are lamprey-like, filled with serrated, jawless teeth.
Aboleth underbellies were often orange-pink, while their topsides were typically sea-green. A little bit back from the head were four long tentacles, two sprouting from across each other on the top, and two more of the same on the underbelly. Their heads were triangular-shaped, with a spherical, beak-like nose.  Above the nose are their three eyes, each one set atop the other. Tendrils and a few shorter tentacles dangle from the bottom of the head.  Four blue-black slime-secreting orifices line the bottom of their bodies.  The creature's blood is green and thick, oozing like sap.
Aboleths breathe through a thick gray mucus, which covers their body and which they exude from four pulsating organs along their body as they move.  If robbed of the ability to exude this slime, an Aboleth would suffocate in water or on land alike. As such, an Aboleth had to take care of its mucus. Out of the water, an Aboleth's membrane-like skin dries out quickly, but this did not prove fatal in and of itself. Instead, the Aboleth would eventually enter a state of suspended animation, called Long Dreaming. During this process, the so-hampered Aboleth formed a tough, waterproof membrane, like a calcium deposit shell.  Over enough years, this shell would grow harder than steel, forever entombing the dreaming creature.

Aboleths In Io

I admit we haven't seen many Aboleths in the campaigns I've run, though their influence has been felt.  The first one encountered by a group was in the Halls of Pandemonium; a discarded, corrupted psionic entity - a far shadow of its former power.  They ripped it apart, with little understanding of what it was and why it was there, whereas in reality they had released its body from an endless cycle of torment by the ancient dragon, Narizguun.  
  As the world continues to clarify, especially in the Fourth Age (where the Pirate campaigns reside), the Aboleth is a creature that has always had influence in the world of Io.  Its power stretches to the depths of the world's memory, pulling strings and making plans.  A creature of the long game, eternal and dangerous...perhaps some villains have taken notes.  ;)

See you at the table.  
Thoroughly afraid of the water.
- Adamus