The Shellbrood are a dangerous menace, ever present in the depths of the hive. They are left as a deliberate mystery in the narrative, as something for the explorers of the setting to discover for themselves. A few variants will be offered, conflicting stories for a GM to choose between, or perhaps these stories are all red herrings and the truth lies somewhere else.
The basic idea is that these creatures are "xenomorphs". They exist as something other.
While many of the races that have become stranded on the Darkhive share enough biology and social structure that they have been able to intergrate (more or less successfully) into the wider community. The Shellbrood are something else.
Vaguely psychic, or simply psychic at a different frequency to the humanoids of the hive, the Shellbrood seem to tap into the collective dreams of the inhabitants. They don't communicate directly with the other inhabitants of the hive, but pull the symbolism from sentient minds and struggle to replicate it in the real world. This basically means that the warrens of the Shellbrood are nightmarish hive cells, and certain strains of highly mutagenic Shellbrood younglings develop forms that look vaguely like the other races, but not quite right.
No one in the hive knows when the Shellbrood first appeared, records of their presence stretch back generations. Tales of encounters with tentacled monstrosities stretch back further into the archives of certain hive libraries, and these are generally assumed to be evidence of Shellbrood who might have mimicked the forms of another race who once inhabited the hive, or maybe these records archive the original forms of the 'Brood.
There seem to be at least six types of Shellbrood. These are distinct groups that hold specific roles in Shellbrood society. Most of the Shellbrood look insectoid/invertebrate with hard carapaces, snaking tentacles, and elements the don't seem to make sense in strictly biological entities.
Scouts - The scouts generally look spidery, swarming from bulging cocoons in their thousands, starting no more than an inch round (from footpad to footpad, half an inch wide thorax), but potentially growing up to two feet wide and long (ten spindly legs each up to a foot long, with a six inch wide thorax). These creatures crawl all over the warrens of the Shellbrood and often up to a few hexagonal cells beyond them. If left unchecked, scouts make way for workers who convert these cells into new parts of the warren.
Workers - Workers look more like octopi with fragments of carapace on their central body and segments of their tentacles. They tend to be larger than scouts, with the smallest recorded workers having tentacles a couple of inches long, whole the largest can reach a tentacles up to a few feet long. Workers consume organic matter, such as animal bodies and most forms of fungus, and secrete a hard resin in the familiar biomechanical designs that signify the territories and warrens of the Shellbrood. It is said that some non-shellbrood inhabitants of the hive have managed to "tame" workers, and use them to build structures of their own.
Harvesters - Harvesters seem vaguely like hermit crabs, living inside the skulls of other cretures from across the hive. Those courageous (stupid) explorers who delve into the warrens of the Shellbrood claim that the Harvesters tend to gardens of fungus that would make many of the Panaho envious, taking the biomatter of the fungus and turning it into a glowing liquid that seems to sustain the Shellbrood Breeders and Broodqueens (while being highly toxic to most other races across the hive). Skulls like those inhabited by the Harvesters are commonly found embedded into warren walls and it is hypothesised that they give themselves (and their skulls) when near the end of their lives to the workers to become an eternal part of the warren complex.
Warriors - Warriors look like a cross between praying mantises and skeletons, with razor sharp claws and bony serrations on their limbs. Warriors begin small but grow rapidly into a stabilised form larger than most humanoid inhabitants of the hive (maybe 8-9 ft tall), they are fast killing machines called into action when a scout or worker is disturbed by a non-Shellbrood. Otherwise they defend the inner parts of Shellbrood warrens when outsiders stray inside.
Breeders - Looking like giant leeches (anywhere from a few feet long to a dozen or more), but with the back half of their bodies a translucent sac containing dozens (if not hundreds) of eggs in various sizes, the Breeders are one of the central elements of any warren. Most warrens seem to have up to half a dozen generally stationary breeders, only rarely are they seen moving, and when they do move it is rarely beyond a snails pace (barely a metre or so per minute). While most Shellbrood castes are able to produce only a few offspring from their own caste, Breeders seem able to convert their eggs into whatever type is most necessary to the hive at the time. In addition to this, those explorers with psychic potential claim the have encountered "psionic white noise" in the presence of Breeders which disrupts any powers they might have. This psychic effect may explain how a Breeder knows which eggs to produce, and explains why certain explorers comsider the Breeders to be "psychic hubs and routers" for the warren. Many suffer nightmares for days after encountering a Breeder.
Broodqueens - While no scholar has ever been able to confirm this, it seems that Broodqueens are an evolved form of Breeder. Every warren has a Broodqueen, and this means there are at least a dozen of them. The main visible difference between a Broodqueen and a Breeder comes from dozens of tentacles and eyes across the flesh of the creature. But in addition to this, Broodqueens are able to shift their forms like an amoeba to form approximations of other shapes. Some claim to have seen Broodqueens with constantly mumbling humanoid faces on their bodies (which turn into shrieking faces when they get agitated), others claim to have encountered Broodqueens with entire humanoid torsos growing out of their fleshy forms...sometimes even manipulating humanoid technologies through these bodily extensions.
Debates rage about other possible castes of Shellbrood, with the following two variants the most commonly accepted of these.
Bloodworms - Those explorers who seek to learn about the Shellbrood warrens (and those warriors who attempt to eradicate them), often return with large leech-like creatures attached to their bodies. Such creatures aren't known to be a specific caste of Shellbrood, and might simply be an organism that lives symbiotically with them. Bloodworms have natural anti-coagulants in their saliva, and removing them often results in major bloodloss (which can prove lethal to a male). But even though there is a risk in removing them, this is always preferable. Those who do not have bloodworms removed often claim to have regular nightmares, and have been known to walk in their sleep...there have even been circumstances where somnambulists under the influence of Bloodworms have opened otherwise impregnable defences to advancing Warrior Shellbrood, laying waste to entire villages as a result.
Facelings - Few would ever dare to claim that the purest of pure bred Kithlings are a caste of Shellbrood, but the limited shapeshifting potential of the Kithlings and the form mimicry of the Broodqueens seems to have a degree of commonality. In addition to this, wherever there is a concentration of strong Kithling blood near a warren, a type of creature referred to as a "Faceling" occasionally appears. In their "natural" form, a Faceling looks like a humanoid sized member of the Warrior caste, with two arms, two legs and a more skull-like face. What makes Facelings most dangerous is the fact that they secrete a fleshy substance across their bodies which is capable of mimicking the exact texture and colour of the dominant races nearby. Only two things readily give away a "Faceling"; first, the fact that they seem unable to speak beyond unintelligible gibberish that sound like words in another language, but makes sense to no-one, second, the fleshy coating is quite fragile and any physical damage has a tendency to reveal the insectoid carapace beneath.
If you don't get the musical reference on this one... also consider the cover on the album this song appears on... I might have considered something from Emerson, Lake and Palmer's "Brain Salad Surgery" but none of them are female.