The insidious threat of the Genestealer has spread like a plague across the galaxy, infecting thousands of worlds with the taint of the Tyranid race. Often the precursor to a major invasion, the extermination of these highly dangerous aliens wherever they are found is of utmost importance to the Imperium.
As the first of the Tyranid organisms known to have been encountered by the Imperium, there is more documented information on the Genestealers' nature than any other species populating the grotesque armies of the hive fleets. The majority of Genestealers encountered thus far have exhibited similar behavioural patterns and near identical physiology. However, certain specimens have been found to have evolutionary biomorphs, showing categorically that Genestealers do not conform to a strict biological template. One of their most disturbing traits, that which gave rise to their name, is the ability to assimilate the genetic pattern of their host species.
Genestealers, like virtually all Tyranid organisms, are characterised by their six limbs and resilient carapace. They are bipedal, able to move with lightning speed on their reverse-jointed, clawed lower limbs. The upper sets of limbs are distinctly different, the foremost pair ending in razor-sharp claws capable of slicing through even Tactical Dreadnought armour (ref. MGP.Macragge. 1stComp.qv). Their secondary limbs are typically shaped like gnarled hands, allowing the Genestealer to manipulate objects, climb and even operate simple devices such as touch-panels. The number of digits differs depending on the parent organism. Despite their dexterity, these secondary limbs are still more than capable of ripping a limb from its socket. The thickly-muscled tail appears to be vestigial, although could aid the balance and agility shown by all variations of the species.
The colour of the Genestealer varies depending on its parent hive fleet, specimens having been reported ranging from bone-white to jet black. Perhaps the most successful strain of Genestealers is typified by a blue-indigo coloration. This strain has been encountered on numerous space hulks and is possibly specifically nomadic; bio-engineered purely to infect new hosts.
A Genestealer's head is characteristically bulbous and houses a disproportionately large brain for such a single-minded creature. Its jaws are lined with viciously sharp teeth, all designed for ripping and tearing; like other Tyranids they only have incisors, not molars.
Within its maw is its reproductive apparatus, the ovipostor, through which the Genestealer implants its seed into a host. This takes the form of a thick, flexible proboscis that ends in a diamond-hard tip able to break through bone with relative ease. The throat of the Genestealer is thick with muscle allowing it to shoot the ovipostor from its mouth with incredible force and speed.
Their carapace, along with the density of their internal skeleton, typically thickens with body mass; a Genestealer whose host was Orkoid will typically be tougher than one born of Eldar gene-stock. Underneath this is the fibrous muscular sheath that can be compared to standard flak armour in terms of durability. These layers provide a considerable degree of protection; combined with their naturally tough physique it is possible for a Genestealer to charge headlong through a volley of lasgun fire and survive. As with other Tyranid organisms, they typically have an open circulatory system with haemolymph flooding the intercellular spaces. This system is host to unnumbered phage cells, believed by the Magos Biologis of New Hallefuss research facility to be digestive systems. These allow them to feed on the nutrient-rich end product of a Tyranid invasion. Closer investigation shows the phage cells to have a dual purpose, acting in a manner similar to fibrinogens in the human bloodstream and clotting the liquids that seep from any wounds that the Genestealer has suffered (ref.MBfl277.anticoagulant.hellfire.qv).
Another physiological anomaly that Genestealers display is the redundant respiratory and circulatory systems irfherited from their host. Furthermore, they exhibit vestigial digestive systems. Some specimens even have complete stomachs, although these are superfluous given the efficiency of the phage cell. These expendable systems allow the Genestealer to sustain considerable non-lethal damage and still function. It is widely known by Imperial forces that these coming into conflict with Genestealers should direct their fire towards the thorax and abdomen of the beasts, as even with several extremities missing they are still highly dangerous opponents.
Thanks to the capture of live Tyranid specimens by the Draco Legion of the Adeptus Astartes, it is known that Genestealers are able to feel pain and react adversely to its application, either becoming incredibly aggressive or cowed into temporary submission. Genestealers have a tremendous tolerance for cold, allowing them to survive in deep space, hidden within the bowels of the space hulks they typically infest. They are even able to survive in a vacuum for a short space of time. To truly exploit the space detritus it inhabits, a Genestealer has great longevity and also the ability to endure long periods without nourishment. A Genestealer can also enter a torpid state at will, lowering its metabolic rate dramatically. Therefore, they are able to survive long periods of inactivity and hardship until new prey enters their lair.
Due to the fact that the Genestealers were the first of the Tyranid organisms to be encountered, it was widely believed that the site of their discovery, the moons of Ymgarl, was their home world. The initial description of the Genestealer, by Sergeant Justus Miale, describes the creature as having six limbs, two of which culminated in clawed arms, aggressive and bipedal. The Ymgarl creature had a long muscular tail and a tough sinuous body. Its face was a mass of sensitive feelers and tendrils surrounding a circular mouth filled with inward-pointing teeth, although the ovipostor is practically identical.
Extensive post-exterminatus archaeology on the now barren surface of Ymgarl has revealed fossils of the Csith, a large leech-like organism that could well have been the preferred host for the Ymgarl Genestealer, thus explaining the differences in physiology.
Originally, Genestealers were thought to be indigenous lifeforms and, although the aliens were deadly they were believed to pose no real threat to the Imperium. Hence no active xenocidal campaign was undertaken at the time. However, it has since been ascertained that this is not the case. Genestealers were found in a number of distant locations in practically every Segmentum over the following centuries, including aboard an increasing number of drifting space hulks.
Shortly after this discovery, Hive Fleet Behemoth invaded from the galactic south-east. Entire armies of Genestealers were seen in the midst of the Tyranid forces at the Battle of Macragge, attacking in unstoppable waves, time and time again overwhelming even the most well-defended locations with their speed and unwavering ferocity.
A vigorous investigation by the Inquisition, commissioned by Inquisitor Kryptmann, subsequently discovered that the Genestealer is a vanguard organism; the precursor to a full-scale Tyranid attack. This explained their presence aboard the space hulks, immense agglomerations of debris and wrecked craft that float ponderously through the warp, periodically reappearing in real space. Most space hulks are empty, lifeless shells. However, a significant number have been found to contain rare and archaic machinery, or relics from the Dark Age of Technology. Such a prize is without price, and no amount of risk is unjustified in its retrieval. As a result, Explorators, treasure hunters and pioneers have been known to board a space hulk in the hope of uncovering such devices. Unfortunately they often uncover nests of Genestealers, which proceed to impregnate them before allowing them to return to civilised worlds. In this manner the Genestealer spreads the Tyranid infection across the galaxy.
Genestealers do not rely purely on their deadly speed and vicious, ripping claws to defeat their enemies. They are possessed of considerable intelligence, comparable to that of Lupus Fenrisii, and are able to coordinate stealthy attacks and set traps when hunting their prey. It is postulated that they convey information telepathically since no other form of communication has been observed thus far. This 'brood intelligence' is thought to be akin to the gestalt consciousness of the Hive Mind, only on a far smaller scale. The Genestealer brood can therefore act as an autonomous unit, able to function light years away from the synapse control of the larger Tyranid creatures.
Much of the Imperium's information on the combat abilities of the Genestealer has been supplied by the 1st Company of the Blood Angels. They have performed numerous expeditions into the depths of space hulks such as Spawn of Execration, Charybdis, Immeasurable Hatred, Sin of Damnation and Harbinger of Despair. Decorated with the Blood Star after his success in leading missions into the heart of two of the aforementioned hulks, Captain Lorenzo of the Blood Angels has filed comprehensive reports on the tactics used by these aliens and the lethal threat they pose.
The Genestealer will not hibernate until it is absolutely familiar with its surroundings, including in-depth knowledge of the ventilation systems, sewers and other crawlways that surround the space hulk's corridors. In this way they can surround and ambush their prey, whose knowledge of the spaceship's labyrinthine passageways is often woefully inadequate. This allows them to close incredibly rapidly, denying their foe the opportunity to cut them down with ranged weaponry. Once the Genestealer is in close combat, it utilises its clawed forelimbs to rip apart its opponents. The three claws on each forelimb are incredibly sharp and diamond-hard, able to slice through bulkheads and cut through the thickest armour. Combined with the awesome strength afforded by the efficiency of the Genestealer's musculature, it is quite feasible for a Genestealer to rip its way through the side of a Chimera to get to the troops inside. The survivors of a Genestealer attack are inevitably heavily armed, forewarned and well-trained, or a host carrying purestrain seed.
Although the characteristic claws of the Genestealer are its primary weapons, certain variations in the xenomorph's form have been reported across the galaxy. Long, stabbing talons occasionally replace the Genestealer's secondary limbs, and several specimens have been found on Ork-infested hulks with thicker carapaces. In 234921.M41, a Genestealer was captured that carried virulent inorganic poisons and haemotoxins in sac-like pouches on is arms. Another known genus can shoot thick, barbed strands of sinew into their victims to keep them from moving freely as the Genestealer closes in for the kill. These 'flesh hooks' are dispatched from the ribcage by a sharp intercostal muscle spasm, and can also aid the xenomorc-h in climbing walls and other vertical surfaces. Presumably these traits are either inherited in part from the host species, or bioengineered by the Hive Mind in its eternal quest for ever more deadly soldier-organisms.
Genestealers have evolved specifically to fill a close assault niche. No specimens have been reported as possessing any of the biological weapon-symbiotes typical of Tyranid Warriors. As with all of the Hive Mind species, what technology they do have is purely biological, advances such as the aforementioned flesh hooks more than compensating for their lack of conventional equipment.
The Imperium no longer underestimates the threat posed by Genestealers, and their extermination has been given the highest priority.
If a single Genestealer or even an organism carrying the alien's seed is allowed to reach an inhabited world, they can infect its populace with alarming speed, quickly spreading genetic corruption and unthinking devotion to the father organism (ref.MD346.Coratinorum.int.qv). This will eventually result in a clique of hybridised slaves, ranging from a small coterie to a major rebellion. Once this has attained significant size, the collective psychic resonations of the cult's members shine like a beacon through the warp, attracting the rest of the hive fleet. In this manner the Genestealers ensure that their parent hive is guided to areas of plentiful biomass and abundance of suitable prey species. Without a populous planet the cult cannot spread; the psychic signature of the Genestealers remains dim, and the hive fleet does not expend valuable energies journeying to barren or sparsely populated systems.
The efficiency of this approach is without question, as Genestealers have been encountered across the length and breadth of the galaxy. The Imperium has long since realised that the destruction of Genestealer infestations wherever they are found is of the greatest importance, as it could feasibly prevent entire hive fleets from descending upon a system. Space hulks are boarded whenever they appear by squads of Space Marine Terminators, who will not stop until they have either eradicated every sign of Genestealer infection from the craft or have all perished in the attempt. The High Lords of Terra have issued an unprecedented eighty-two decrees ordering the Genestealers' extinction via the most extreme methods available to them. In point of fact, Exterminatus, whereby all signs of life are scoured from a planet via the use of powerful cyclonic torpedoes, is thought to be the only truly effective method of dealing with a Genestealer infestation. The Imperium shows no hesitation in the use of this apocalyptic tactic. Even an unconfirmed rumour is enough to merit an extended investigation, but one confirmed sighting is enough to warrant Exterminatus. In 345087.M39, the Salamanders Chapter of the Adeptus Astartes finally used this method to cleanse Ymgarl and its moons of any sign of life, but by then this was just one world among hundreds that harboured the alien threat.
When a Tyranid invasion is underway, the military might of entire Segmentums is mobilised to repel it in all-out war. Only with total commitment, sacrifice and dedication can Imperial troops ever hope to stem the tide of a hive fleet. The Tyranids are so pure of purpose that they cannot ever be bargained with, diverted or exploited, and Genestealers exemplify this.
Amongst such a single-minded race as the Tyranids, Genestealers have an unusually complex and advanced social structure once their infection has been allowed to spread. At a fundamental level, a purestrain Genestealer brood is uncomplicated by such nuances, with no one alien having any more importance than another. Just as a group of biological cells or a swarm of insects, they function in perfect unison, lacking the concept of self. However, if a single Genestealer has successfully impregnated a target on a populated world, a bizarre and perverse family structure grows up around it, ensuring its safety and ascendancy as the nexus of the resultant cult.
The Genestealer responsible for the initial infection implants its egg into a Human, or indeed any other creature. The Genestealer's gaze has a hypnotic effect on its prey, allowing it to close in and implant an egg beneath the skin. In fact 'egg' is something of a misnomer; it actually acts more like a cancer, altering the host's genetic structure and in particular its reproductive system. The victim itself is not subject to any debilitating effects, actually increasing in strength and health as the infection takes root. When the victim mates, its offspring will be hybrids - part Human, part Genestealer. The hybrid child does not consume its parent, as with more inefficient parasites. One of the changes wrought by the infection is the absorption of part of the host creature's brain. During this foul process the parent is shorn of any free will, reduced to the role of slave to its unborn child. Once the infant is born, the parent will go to any length, even suicide, to protect its offspring. In the parent's mind, the child is hale and hearty, a specimen of physical perfection. In reality, it is a repugnant, mewling crossbreed, discoloured features twisted into a fanged mask. This mockery of the family unit is incredibly disturbing - a message from the Great Devourer that even ties of kith and kin will be consumed and assimilated by the Hive Mind.
The offspring of these unholy unions then interbreed, flocking together in the darkness until a sizeable cult surrounds the purestrain Genestealer at its centre. This patriarchal figure grows older and larger on the adulation and support of its cult, becoming even more grotesque and developing a level of intelligence comparable to that of a human. More and more hybrids are born, with varying degrees of genetic corruption. Some could even pass for human, although they have a marked tendency toward hairlessness and heavy bone structure, and their stare is extremely unsettling. Others may bear close resemblance to their forefather, and a rare few are born as true Genestealers. Needless to say, the degree of mutation is not an issue among the cultists; the procreation of their hideous group overcoming all feelings of revulsion or propriety.
Many of these hybrids are able to exercise the human intelligence stolen from their genestock, learning quickly how to utilise conventional weaponry and infiltrating military and political institutions to further the aims of the sect. Worshipping their Patriarch as a god, they stop at nothing in their corruption of the dominant command structure. To this end, leaders of the cult direct their purestrain brood-kin to impregnate influential figures within the local authorities and planetary defence forces. Those implanted subsequently lose all free will, lying, murdering and blackmailing to further their power, the better to tear down organisations from within when their true masters descend from the stars.
When the cult has grown to significant size, the psychic beacon that emanates from the cult's Patriarch ensures that a hive fleet will finally descend upon the doomed world. As the cult comes into range of the Hive Queen's psychic control, it becomes utterly subservient to the Tyranid invasion, and the underground cult will explode in bloody and violent revolution. This uprising is sometimes contained by the ruling forces, but usually by the time the Tyranid fleet arrives, the victim world's defences are rife with confusion and insurrection. The destruction of the cult is of no consequence to the Hive Mind, but this devastating preliminary attack can cripple the world's defences even before the first mycetic spore enters the atmosphere.
Brood: The brood consists of between 6 and 12 Genestealers.
Bio-weapons: Genestealers have rending claws. Depending on the host species, Genestealers show occasional mutability and the whole brood may be equipped with up to one bio-weapon or biomorph enhancement chosen from the following list at the points cost indicated per model.
Infiltrate: Genestealers are intelligent and stealthy. They will take advantage of any opportunity to creep closer to their prey before springing ferociously into the attack. To represent this, Genestealers may infiltrate in scenarios where the Infiltrate scenario special rule is being used. If the mission does not allow units to use the Infiltrate rules then the Genestealers must set up normally with the rest of the army.
Brood Telepathy: Genestealers have their own brood telepathy which allows them to function independently without the influence of the hive mind. Because of this, Genestealer broods outside the range of any Synapse creatures do not use the Instinctive Behaviour rules and instead take Morale checks and Pinning tests just like ordinary troops.