The aboleth was created by David «Zeb» Cook for the first edition of Advanced Dungeons & Dragons, and they have appeared in every edition of the game since. In the «Lords of Madness» sourcebook, they are featured in their own chapter. According to the information within that chapter, aboleths are some of the most ancient beings in existence. You can find the official entry with stats about the aboleth for 5e in D&D Beyond.
I’d like to dedicate this entry to my long-time friend and fratellino Rafael, aka. «Lord Archaon», since I know these creatures are among his favorite monsters to use in his campaigns.
Aboleths are fish-like amphibians, twenty feet long and weighing about 6,500 pounds; they continue to grow as they age, however, and some fantastically ancient specimens reach much longer lengths. They are a kind of hybrid of fish and eel with some insectoid and annelid qualities; they have long, tubular bodies, like that of an eel, but fish-like tails at the end and two fins near the head and a dorsal fin near the back. Their bodies are also segmented, which is a worm or insect-like characteristic. A little bit back from the head are four long tentacles, two on each side, two on the topside and two on the underbelly. Their heads are roughly triangle shaped, with a spherical, somewhat 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.
Aboleths have powerful psionic powers, being natural psions like the illithids. However, the aboleth, like the other archetypal aberrations, has a much more fearsome ability: aboleths secrete a viscous grey fluid, much like mucus, which brings about a terrible transformation in air-breathing creatures. The skin of the victim is transformed into a membrane which allows it to breathe in water, but robs the creature of air-breathing. This allows the aboleth to keep slaves, which it dominates and keeps captive with its mind.
Another strange feature of aboleths is their memory. An aboleth is born with a racial memory, each individual inheriting the memories of its ancestors. Furthermore, it assimilates the memories of those it consumes. An aboleth’s memories are stored in an ever-growing part of its brain which extends down its back as it ages. Aboleths enjoy spending time lost in particularly fine memories of their ancestors, and if they have nothing better to do, they may relive entire portions of their lives.
Aboleths do not die of old age, living indefinitely barring death from violence or disease.
Aboleths are utterly self-centered as a race; they know they were among the first beings in existence, and see all else as theirs. Their enmity towards other races stems in part from their perception that these «upstart» races have stolen what is rightfully the aboleths’. All that stops them from conquering the surface is their weakness on land and the fact that they would rather enjoy themselves than waste time subduing feeble creatures such as humans. By contrast they are greatly unsettled by the Illithids due to their lack of information over that race’s creation.
Aboleth cities are vast affairs of bizarre and alien architecture, located deep underwater. The Shape of Water, located in the Underdark’s Glimmersea, is the largest known Aboleth city. This is where the leaders of the race reside and hold council.
Aboleths have no gods. While they acknowledge the presence and power of gods, they have memories of a time long before any modern gods were worshipped and recall such gods’ birth and often demise. They are not concerned with an afterlife since they intend to live forever, considering death a failure. They do have a certain respect and reverence for the ancient beings known as the «Elder Evils» Bolothamogg, Holashner, Piscaethces, Shothotugg, and Y’chak, based on the Cthulhu Mythos of H. P. Lovecraft. According to the 2nd edition box set Night Below they do have a god, known as the Blood Queen. Other sources also indicate that a minority of them worship Juiblex.
In the 4e Underdark sourcebook, it is noted that some sages are unsure whether aboleths have a true self-awareness, or instead have a form of psyche utterly alien to all existence. It is also noted that aboleths were originally native to the Far Realm, and they actually seek to have not just the world, but the totality of the Astral Sea and Elemental Chaos as well, subsumed by the Far Realm.
An aboleth brood consists of a parent and one to three offspring. Though the offspring are as large and as strong as the parent, they defer to the parent in all matters and obey it implicitly. Aboleth have both male and female sexual organs. A mature aboleth reproduces once every five years by concealing itself in a cavern or other remote area, then laying a single egg and covering it in slime. The parent aboleth guards the egg while the embryo grows and develops, a process that takes about five years. A newborn aboleth takes about 10 years to mature.
The omnivorous aboleth will eat any organic matter, usually algae and micro-organisms, but they are also fond of intelligent prey so they can absorb nutrients and information at the same time. Aboleth have no natural enemies, as even the mightiest marine creatures give them a wide berth.
In the «Eberron Expanded» column about Lords of Madness, Keith Baker adapts the origins of the aboleth into the Eberron mythology: «Most legends speak of the rakshasas as the first fiends and refer to them as the most ancient enemies of the light. But these creatures were not the only children of Khyber: the oceans became home to the aboleths at about the same time as the first rakshasas were created. The rakshasas ruled the land, but the aboleths held sway in the deep waters, unknown to all but a few. Both battled dragons and couatls, and in time, both saw the Elder Evils cast down and bound by the Silver Flame. After this defeat, the aboleths retreated to the depths where no dragon could follow them. In the hundreds of millennia since, they have slowly returned, spreading their influence across the world. Now the aboleths wait for the day when the Silver Flame will gutter and die, allowing the Elder Evils to return and claim the world as their own.»
With this version of Eberron’s history, the aboleths become contemporaries of the Lords of Dust and enemies of the Chamber and the Church of the Silver Flame. Like the dragons and the rakshasas, the aboleths have the perspective required to study and understand the Draconic Prophecy fully, and they have discovered segments of it hidden in the deep waters. Like the rakshasas, the aboleths seek vengeance against the dragons and intend to twist the path of the Draconic Prophecy. As a result, characters working for the Chamber can easily be drawn into conflict with aboleths and their agents.
In your campaign
You can instead use the aboleths as heavily ostracized and isolated monsters, better left alone. They have withdrew themselves into the dark depths of the sea all around the civilized lands, leaving many ruins ashore. These ancient aboleth ruins hold remnants of powerful magic and cursed artifacts linked to the Far Realm.