Mankind cannot live through this. In a single day they have covered the surface of this planet with a flood of living blades and needle-fanged mouths. Kill one and ten take its place. If they are truly without number then our race is doomed to a violent death before every shred of our civilization is scoured away by a force more voracious than the fires of hell themselves! Death! By the Machine God, Death is here!
-The Last Words of Magos Varnak
The Tyranids, also known as The Great Devourer are an extragalactic alien race, whose sole purpose is the consumption of all forms of genetic and biological material in order to evolve and reproduce. Tyranid technology is based entirely on biological engineering. Every function is carried out by living, engineered creatures, each of which collectively forms the Hive Fleet, directed by a single Hive Mind.
The Tyranids are seen as one of the gravest threats to the entire Galaxy. They seek only to consume all organic life and cannot be reasoned with or deterred in this quest. Worse still for the Galaxy, thus far the Tyranid Hive Fleets that have been encountered are merely the furthest stretched tendril of the main invasion fleet that is still traveling in the void of space.
History
I don't know where they come from. It seems highly unlikely that they evolved naturally into what they are now—impossible, given what we know about evolution. Perhaps they are some kind of bioweapon, created millennia ago by madmen for some long-forgotten war. If so, I suspect that their creators soon came to regret their creation. Or perhaps they come straight from the Warp. Possibly the Lords of Chaos themselves cast them out because they were too unpleasant.
-Jarv Advent, Senior Xenobiologist, Inquisitorial Institute of Proctos Minor
The exact origin of the Tyranids themselves is unclear, save the fact that they are not of this galaxy and have only recently arrived here after traveling countless millennia in the intergalactic void. It is unknown which galaxy they originated from, or for how long the Tyranid race has been on its genocidal rampage, but it is believed that it is the Astronomican that is drawing the Tyranid Hive Fleet to threaten the Galaxy. The Tyranids were first attracted to the Milky Way Galaxy when the Xenos communication device known as the Pharos was overloaded in the Battle of Sotha during the Horus Heresy. Indeed their very name is but a title given to them by the Imperium, named after the planet where they were first encountered (Tyran). It is possible that they have been preying on other galaxies since time immemorial. According to another source, they have consumed one thousand galaxies and are responsible for the annihilation of millions of intelligent species.
Biology
The Tyranids do not communicate with the other races of the Galaxy. 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.
The Tyranid forces are constantly changing and evolving at an unnatural speed. It is unlikely that all types of Tyranids have been seen by the Imperium, and less likely still that they will ever all be seen. However a few key traits are identical to nearly all Tyranids, such as tough chitinous armour, a hexapodal anatomy, and multiple redundant organs which make them incredibly difficult to kill.
