Source: The Flooded Cathedral, pg(s). 84
An abhominal has pale, translucent skin and is arched backward, scuttling on its hands and feet. A mass of barbed, black tentacles extends from its exposed abdomen. The midsection of an abhominal rises to a height of about two and a half feet, though its tentacles go much higher than that. It weighs over 250 pounds.
Abhominals bleed a brackish black liquid that permeates their entire bodies, colouring their glistening eyes completely black. Beneath their skin is a hardened carapace, and their hands and feet are covered in fine branching bristles that burrow into surfaces.
Abhominals are a result of a failed experiment in which an aboleth employed ancient Azlanti technology to alter sentient humanoids. The test subjects did not survive the gruelling transformation, but were animated as undead bound to the aboleth's will. The very first abhominal raved like a rabid beast, lashing out at anything that came near unless its creator exerted psychic influence over it. When more abhominals were created, they started to exhibit more intricate behaviour, and the newly formed hive mind became controllable.
The flawed experiment that results in the creation of abhominals always ends with the test subjects' death. In their final moments, they are pale, feverish and bloated. Their nascent telepathy whispers dissonant echoes of the hive mind reaching out to them. Upon death, their bodies convulse, limbs snapping at weird angles, black bile spills from their mouths, tentacles burst out of the abdomen, and the newborn abhominal screams out a telepathic call to others.
Abhominals coordinate their movements with telepathy when hunting. If one notices an intruder, then all abhominals within reach are telepathically notified as well. Their alghollthu master can assume a more direct command when needed.
Abhominals nest high above ground, preferring areas with passages and openings leading to areas below, forming a central hive and outlying nests between which individuals constantly move. Abhominals clump together and spread from the ceiling downward, their tentacles entwining like a web between them.
Abhominals are quite dependent on the telepathic connection that binds them together and go berserk if separated from the hive mind. In sufficiently large numbers, their thoughts begin to harmonise and exhibit cognisance that might threaten aboleth control. A large, unsupervised hive might find a way to make more abhominals of their own, although an aboleth would never allow such a thing and frequently has to affirm its dominance and maintain its psychic link.
Abhominals follow the directions of their aboleth master if they have one; if not, they will give in to their hunger and attack all living creatures, while other undead are left alone and might form mutually beneficial partnerships with them.