A Genestealer is a species of Tyranid used as the ultimate shock trooper. Their purpose is to advance ahead of a Hive Fleet and pinpoint potential planets for the Tyranids to devour. They are amongst the most deadly creatures in the Galaxy, combining high cunning, lightning-fast reactions and movement, with large, extremely sharp claws that can rip through the toughest armour in seconds.[1b]
Genestealers were the first Tyranid species to be encountered by the Imperium. But before the Imperium had knowledge of the Tyranids as a species, they believed Genestealers were a separate species indigenous to the moons of Ymgarl which spread to other planets by infiltrating passing cargo ships and derelict Space Hulks.[1a] It is believed Genestealers were introduced to the moons of Ymgarl in M35 from Hive Fleet Tiamet in the Tiamet System, carried inside the Imperial vessels that were sent to destroy the system.[2a]
When the Battle for Macragge with Hive Fleet Behemoth took place, Technomagi spent years classifying the left-behind bodies, including Genestealers used as Tyranid shock troops. Further investigation confirmed that all Genestealers were in fact Tyranids.[1a]
The truth is that infiltrating Genestealers had been working their way into the galaxy for thousands of years prior to the invasion of the Hive Fleets proper.
|Ordo Xenos Departmento Analyticus Record |
|Common Title||Stealer, Leech, Sewerstalker, Scuttler, Clawfiend, Ghost [2c]|
|Species Name||Corporaptor hominis, Corporaptor ymgarli , Vermis (Tyrannus) Furii |
|Average Weight||0.3 tonnes|
Genestealers are stronger and hardier than Humans, able to withstand even the most hostile environments unprotected, including hard vacuum, and are capable of living for centuries . They are a highly intelligent species with a disproportionately large and adaptive brain that is housed in a bulbous head [1b]. They combine extraordinary intelligence and subtlety of mind with remarkable strength and quickness of body.
Genestealers are bipedal, standing in a perpetual crouch, and have two sets of arms, one equipped with Human-like hands, one with powerful ripping claws. Their dense musculature and the distending hinge of their jaw allows the generation of incredible bite-pressure.
Their olfactory organ is linked at its base to the rear of their palate, combining their scent and taste for possible use as a sophisticated air-analysis method. They also display respiratory gill systems along the dermis/valves between their fused exo-ribs. This system is then somehow linked to breathing holes on their limbs and cranium.
However these characteristics may not be general across all Genestealers, as they are one of the most mutable Tyranid species and often exhibit a wide and varying range of different traits. It has been hypothesised that these traits manifest depending on the particular host species the infection discovers; but this theory has been disregarded by some experts, citing that the traits are more likely different Genestealer subspecies altogether, which are separate from the host.[2c]
Purestrain Genestealers are Genestealers which have been spawned not through the process of infecting hosts but rather by the hive ships themselves in great bio-factories and birthing vats. Not polluted or altered by “inferior” genetic material, they are markedly stronger and faster than a normal Genestealer and possess a wider variety of adaptive biomorphs. This is intentional, as Purestrain Genestealers are the creatures from which all other Genestealers are born, and even a single Purestrain Genestealer can create a vast brood from a local population given time and enough raw material.
Possibly the most terrifying aspect of Genestealers is their insidious method of reproduction. Genestealers reproduce completely separate from a Hive Fleet, unlike the majority of other Tyranid creatures. Through the use of a tube-like tongue called an Ovipositor, they insert a tiny embryonic organism containing their own genetic material into a host victim. Over time this organism alters the host's genetic code, and their consciousness is also completely suppressed by a localised version of the Hive Mind, known as the Brood Mind, becoming a virtual slave. Any offspring born by the host will be horrific Genestealer Hybrids, which the host will be psychically mesmerised to care for, giving rise to the Genestealer Cults that grow in secret within human and other societies. 
Genestealers have an interlinking telepathy similar to the greater Hive Mind which is generated by the entire Tyranid race. This Brood Mind is smaller and more localised than the Hive Mind and is used to exert complete control over their host victims  and also for communication . It also acts as a psychic beacon to draw a Hive Fleet towards the infestation in order to devour the planet.
The relationship, if any, between the Broodlord and Genestealer Patriarch is undetermined.
A Genestealer Patriarch is the founder and leader of a Genestealer Cult.
Genestealers form the first wave of a Tyranid invasion, either infiltrating a suitable world hundreds of years before the actual invasion, or in the more short term, where they are deployed onto a planet by Mycetic Spores from the hive fleet. In both cases they pave the way for the invasion by weakening the planet's defences.[2b]
Once upon a new world, the Genestealers seek out all forms of life, attempting to home in on species of a highly organised nature. Genestealers at first concentrate on infecting host victims, who go on to breed Genestealer/host hybrids and begin a generational cycle of new hybrids increasingly like the host species. Over decades, the hybrids will spread through a society and a hidden cult is established within the host's society. All members of the Genestealer cult are psychically linked and controlled by the original Genestealer. The cult is totally devoted to gaining political power within the society, often cloaking itself in the guise of a legitimate religion, while infecting suitable hosts, prospering and multiplying, and hiding their true nature from the broader society.
A full Genestealer starts the infection cycle by inserting their genetic material into a host victim. As the infestation spreads and the cult grows, the first Genestealer takes the role of a Genestealer Patriarch and often grows larger into an obese and psychic monstrosity.
Infection Generation- Contagii
The host victims, or Contagii, return to their own societies and begin to have an urgent need to find a mate and begin a family. This mate also becomes infected with Genestealer DNA. Contagii still physically resemble other members of their species, though they may have a slight blue or mauve pallor to their skin and can be psychically controlled by the Patriach to become mindless slaves, making them the perfect infiltrators.
1st Generation - Maelignaci
As the contagii mate, the Genestealer genetic material passes onto their offspring, the Maelignaci. The Maelignaci are animals, slow, clumsy and moronic. Within the cult they generally regarded as expendable shock troops. They often possess three or even four arms, and are unable to use their host species technology or weaponry.
2nd Generation - Hybrid
The Hybrid offspring of the Maelignaci are born unique with no two alike. However this generation can be selectively bred, using their parents like live-stock, and is often manipulated for traits such speed, strength or aggression. They bear more resemblance to their host species than a Genestealer, but are still unable to use their host's technology.
3rd Generation - True Hybrid
The next generation are True Hybrids and can be mistaken for a member of their host species, if they wear disguising clothes, or stay in dark areas, etc. Some of this generation are capable of using weapons of their host species.
4th Generation - Primacii
The fourth generation, the Primacii, are almost indistinguishable from their host species, only with slight physical deformities. Some still sport give-away signs such as being bald . They are able to blend in to the host society and work to lure, kidnap, or do whatever it takes to provide more hosts. Some rare individuals of this generation have psychic powers and become a Genestealer Magus.
5th Generation - Purii
Paradoxically, the Genestealer's DNA reemerges in the fifth generation, giving birth to a full Genestealer, albeit one showing some physical characteristics of the host species.[1b] Thus the cycle can begin anew, as the new Genestealers have the potential to infiltrate other worlds and become a Patriarch.
Hive Fleet Invasion
The Brood Mind of a Genestealer infestation acts as a psychic beacon to draw in a Tyranid Hive Fleet. When a Hive Fleet comes within a few hundred light years of an infected world, a sublimiminal reaction is triggered within the Brood Mind which forces the Genestealers into a frenzy of killing and breeding, thus collapsing the society in which they were hidden and making way for the full Tyranid invasion.
However Genestealers are notoriously independent from the Hive Mind and have a great will to survive. During a planetary invasion Genestealers wanting to survive are innately driven onwards and outwards from the Hive Fleet by infiltrating fleeing ships. Or, after the invasion, the Genestealers are reabsorbed into the Hive Fleet as pure bio-matter like all other life is. The infiltrating Genestealers go on to infest more worlds and then in turn lead the Hive Fleet to them. No one amongst the Imperium knows how deep and widespread the Genestealer infestation of the galaxy has become.[1b]
- 1: Codex: Tyranids (5th Edition)
- 2: Codex: Tyranids (4th Edition)
- 3: Space Hulk Mission Book (3rd Edition), pgs. 46-47
- 4: Xenology, [Needs Citation]
- 5: Imperial Armour Volume Four - The Anphelion Project, [Needs Citation]
- 6: White Dwarf 114 (UK), White Dwarf 115 (UK), Genestealers by Paul Murphy
- 7: Deathwatch: Mark of the Xenos, pg. 39
- Codex: Tyranids (3rd Edition)
- Deathwatch (Graphic Novel)
- Space Hulk (Game)
- Bringers of Death (Anthology)
- For The Emperor (Novel) by Sandy Mitchell
- Genestealer (Game)
|Command||Hive Tyrant • Broodlord|
|Elites||Hive Guard • Lictor • Malanthrope • Pyrovore • Ravener • Venomthrope • Tyranid Warrior/Tyranid Prime • Tyrant Guard • Zoanthrope/Neurothrope|
|Basic||Genestealer/Ymgarl Genestealer • Hormagaunt • Spinegaunt • Termagant • Ripper|
|Heavy||Biovore • Carnifex • Exocrine • Haruspex • Hierodule • Maleceptor • Mawloc • Tervigon • Toxicrene • Trygon/Trygon Prime • Tyrannofex • Dimachaeron • Sporocyst|
|Airborne||Gargoyle • Harpy • Harridan • Hive Crone • Shrike • Sky-Slasher • Tyrannocyte|
|Bio-Titans||Dominatrix • Hierophant|
|Spores||Mieotic Spore • Mycetic Spore • Spore Mine • Mucolid Spore|
|Unique Tyranids||Swarmlord • Death Leaper • Doom of Malan'tai • Old One Eye • Parasite of Mortrex • Red Terror|