As the Tyranid Hive Fleet nears the doomed world, the Genestealer Cult, potentially numbering in the millions by this point, goes into action, instigating a worldwide rebellion with the goal of weakening the planet's defences against the approaching hive fleet. The synapse creatures that channel the commands of the Hive Mind are mostly, if not all, potent individual psykers. How any of this is done without drawing the attention of countless daemons of Chaos or the other psychic predatory entities of the Warp into the midst of the Tyranid swarm is unknown and possibly unknowable to the humans of the Imperium. Should a psyker attempt to use their otherworldly abilities, the cerebral cacophony worsens even further; the psychic sound of a billion alien thoughts scratch at their mind, and unless they are particularly strong-willed they will be pitched into an insanity where they will repeatedly utter phrases in a tongue too alien to properly pronounce. The Tyranids are a nomadic xenos species governed by a gestalt collective consciousness known as the Hive Mind. The species comprises many different genetically-engineered bioforms, of which the Genestealer is but one. The Tyranids travel in great fleets of gigantic living creatures that serve as spacecraft, each of which is home to countless lesser Tyranid organisms grown in the bubbling organ-sacs of the vessel's reproductive chambers.

Thanks to the capture of live Tyranid specimens by the Draco Legion Chapter 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. In 993.M41 the Tyranids returned to the galaxy with Hive Fleet Kraken which, instead of assaulting its targets as a single massed Hive Fleet, split into countless smaller fleets, each enveloping whole star systems before reinforcements could arrive. Further, depending upon the numbers and types of creatures on the surface, the Hive Mind might still persevere. Even if the Hive Mind were overcome and the majority of the xenos were eliminated, the battles to cleanse the planet completely could continue for Terran millennia. If a permanent spell with ravenous is copied, the copy will have the same value for X, and the token permanent that the spell becomes as it enters the battlefield will enter with X counters. In the latter days of the 41st Millennium the galaxy-wide cataclysm of the Great Rift tore a ragged wound across realspace, signalling the beginning of a new age of darkness, the Era Indomitus of the Age of the Imperium.Given that each of the different Tyranid hive fleets has approached the Milky Way from a different direction, this may imply that the Tyranids have consumed a disturbingly large number of nearby galaxies. The components of a Tyranid hive fleet travel almost exclusively in large groups known as "swarms" that possess specialised biomechanical creatures for destroying and consuming a wide variety of prey life forms.

The Magos Biologis of the Imperium categorises each Tyranid hive fleet as a separate force, an individual entity that competes with other hive fleets for resources. Indeed, each is self-sufficient, appearing to exhibit different strategies and developing unique creatures to overcome its prey. The creatures are clearly aware that they must survive and propagate before the invasion may proceed. These specimens are distinctive from most of the Tyranid horrors in that they are also capable of stable independent action. The vast majority of known Tyranid organisms are incapable of coordinated activity when separated from the psychic emanations of the Hive Mind. the Tyranids slumbered in a state of frozen hibernation, but now they have arrived, they have awoken and they are hungry. A few members of this generation possess psychic powers and become a Magus or Genestealer Primus. Other types of Fourth Generation Hybrids include Clamavi, Nexoses, Sancti, and Biophagi.The massive Warp presence of the Tyranid Hive Mind, however, manifests itself in that realm through the Shadow in the Warp phenomenon. The current collection of Tyranid hive fleets have migrated to the Milky Way Galaxy, presumably after overpopulating or overfeeding in other nearby galaxies of the Local Group. Genestealers are the most insidious of these early invaders. As they scout out the planet, they also infect the planet's fauna with Tyranid genetic material. The corruption of the planet's native gene-code eventually results in hybrid native-Tyranid creatures that are subject to the will of a Broodlord or other representative of the Tyranid Hive Mind. 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). A Genestealer ( Corporaptor hominis) is a bioform of the multispecies Tyranid race that was genetically designed by the Hive Mind for the infiltration of other intelligent species' settled worlds.

Rather, they invade to harvest valuable biomass and feed their insatiable hunger. The Tyranids require an endless supply of food, not only to nourish the hive fleets, but to grow new organisms. Therefore, when a hive fleet invades a planet rich in life, every action of every Tyranid creature is honed to a single goal -- the total and rapid absorption of that world's organic population, ecosystems and bioresources. Eldar know Genestealers by many names, the most famous of which are — The Hider in Plain Sight, Hearth-lurkers, Bloodshadows, Kin-thieves. [12] Images A Genestealer hybrid of four or more generations of consistent Human parentage would pass for a baseline Human on cursory inspection, although a closer look would reveal bluish skin, sharp, pointed teeth and unsettling behaviors. The hive fleet will also seed the planet with numerous Tyranid organisms, which will begin to biologically alter the planet's climate, surface and ecosystems to make it easier to consume.Hive fleet broods are often centred around an alpha predator sometimes mistaken for a Genestealer Patriarch, though this creature is more accurately termed a " Broodlord." This beast is not empowered by the psychic energies of a Genestealer Cult's " Broodmind," but by a single brood of Genestealers. The second generation of hybrids are hunched and stooped -- not in the manner of the old or infirm, but more like pressured springs, ready to explode into movement. Like the generation before them, Second Generation Hybrids, who also serve as Acolyte Hybrids, may have five or six limbs, but their eyes and mouth are more like their Human parents, and they can make themselves understood in Low Gothic. Bio-Titans - The Bio-Titans are enormous organisms, evolved as a response to other races' Titans and other super-heavy warmachines. They are (as of yet) the largest land-based Tyranid creatures encountered. They range from nine to thirty metres tall, and can single-handedly take out an entire squad of Space Marines. Mineral reserves are also depleted, particularly rare ores that may be incorporated into the exoskeletons of the various weapon biomorphs.

In this manner, the planet's natural resources are turned against it, providing additional tools for the swarm. The majority, however, is set aside to feed the Hive Ships orbiting the planet. Those massive creatures generally delay their feeding until after the planet has been secured. The Tyranids are not native to the galaxy; they have journeyed across the unspeakable cold of the void, where time and space conspire to hold the stars apart with inconceivable distances.

Rippers - The Rippers are small, snake-like organisms designed to devour biomatter so that new DNA and nutrients can be absorbed by the Hive ships. They are released in the final stage of a Tyranid invasion.