Aboleths are fishlike 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. Their underbellies are orange-pink, and their topsides sea-green. 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. This same mucus is the only way aboleths breathe at all – if robbed of the ability to extrude aboleth slime, they suffocate in water or on land. Out of the water, an aboleth's membranelike skin dries out quickly, but this does not prove fatal. Instead, the aboleth will eventually enter a state of suspended animation, called long dreaming, a fate considered far worse than simply dying. It forms a tough, waterproof membrane, but, once the membrane is pierced, liquid floods out and death is usually not far off for the aboleth. With the exception of the gained damage reduction and increased armor class, an aboleth out of water is simply a sitting duck.
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 age, and so live on indefinitely barring 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 (though an aboleth is always a fierce opponent) 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 within their own lifetimes. They are not concerned with an afterlife since they intend to live forvever, considering death a failure. They do have a certain respect and reverence for the ancient beings known as the "Elder Evils", 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 4th Edition 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.