Tyranids - Tyranid Combat Doctrine: The Tyranids often attempt to first conquer a planet through subterfuge and infiltration. Space hulks and other derelict voidcraft are often infested with Genestealers, which quickly familiarise themselves with the layout of their new home, then enter a state of self-induced hibernation.When the hulk is eventually discovered by Imperial authorities, the Genestealers attack the exploration crew, sometimes killing some crew members, but always leaving at least one survivor who is implanted with a seed, a form of Genestealer reproductive virus that contains specially-engineered Tyranid DNA.This infestation is never fatal, but instead the virus' genetic material subtly alters the victim's DNA, producing in them a strong urge to mate created by the stimulation of the individual's sex hormones.The Genestealer DNA will be incorporated into the host's germ cells and so any child that results from this union will be a Genestealer hybrid, which now has the complete loyalty of its parents through a psychic bond.The hybrids continue to breed and multiply among the population of the host species, eventually forming a Genestealer Cult. As the cult grows in numbers, it begins to spread its influence throughout the planet's political and social structure, placing its members in positions of power within the government and the military.At the same time, the cult's hybrid members, united by a smaller psychic gestalt known as the Broodmind, begin to emit a collective psychic signal through the Warp that acts as a beacon for the approaching Tyranid hive fleet.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 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.If the hive fleet is victorious, the fleet will descend to the planet, inhaling the atmosphere, drinking the seas, and basically consuming all organic biomass until nothing more than a lifeless ball of rock remains.On the battlefield, Tyranid tactics are based around the notion of superior numbers, as they generally try to outnumber the enemy fifty to one. Tyranids overrun opposition in close combat, closing faster than most armies.They possess few ranged weapons, but their sheer numbers and close combat specialisation makes up for the slaughter if they are led by a capable commander.The Tyranids do not communicate with other intelligent species, and why should they do so? Tyranids are as far above other life forms, such as humanity, just as humanity is above the domesticated livestock it consumes.The Tyranids cannot be reasoned with, appeased or surrendered to. There can be no hope of mercy from such a foe. To face the Tyranids is simply a matter of survival: kill or be consumed.Vast swathes of the galaxy have already been stripped of life, and with every passing Terran year the hive fleets push deeper into regions of populated space. Even as the prey races direct their forces to repel these threats, still more Tyranid fleets approach from the intergalactic void and emerge from their aeons-long slumbers.The thought processes of the Hive Mind are gathering pace as more Tyranids wake and recall the age-old purpose of their kind -- feed, grow, survive, and evolve.