The Tyranids are an extremely hostile alien species from outside the Milky Way galaxy, appearing only recently to plague the sentient races. They form a monstrous superorganism that travels the universe in their great biomechanical fleets, systematically consuming all other biomatter to feed their rapid evolution and reproduction.
The origin of the Tyranids is utterly unknown. They first appeared in the eastern fringes of the galaxy in late M41, emerging from the intergalactic space of the Local Group and heading toward the center of the galaxy. It is speculated their Hive Mind was drawn by the Astronomican's psychic beacon.
First contact with the Tyranids for the Imperium of Man was an attack on a moon called Tyran, which gave the species its Imperial name. This first prong, the Hive Fleet Behemoth, was barely stopped at the Battle of Macragge in 745.M41 by the might of the entire Ultramarines Chapter supported by PDF troops and planetary defense structures, still suffering terrible casualties – including their entire 1st Company to the last man – in the process. And yet, after the battle it was discovered that the Hive Fleet had split and they had only fought one half; however, a major Imperial fleet was able to destroy the other half.
Two centuries later, the Tyranids returned to the galaxy with Hive Fleet Kraken which, instead of throwing one mass of troops against the Imperium, split into countless smaller fleets, each small fleet enveloping whole star systems before reinforcements could arrive. The brunt of this new attack was borne by the Space Marine Chapters known as the Scythes of the Emperor and the Lamenters; the former Chapter was almost completely destroyed. Though the backbone of the Hive Fleet was broken by its defeats at the Battle of Ichar IV and at the Eldar Craftworld Iyanden, the cost to the Imperium was still great.
Only a few years later, Hive Fleet Leviathan unexpectedly appeared from "below" the plane of the galaxy (on the Z axis of the galaxy) and attacked from two points, cutting off large portions of the galaxy from reinforcements. Just as it seemed the defenses of the Segmentum Solar and perhaps Terra itself would be tested, the Tyranids were distracted by being deflected into the star system of a powerful Ork empire. While the Orks are managing to stall the main Tyranid Hive Fleet's advance, they are likely to reemerge from these battles, victorious and stronger than ever after having absorbed potent Orkoid genetic material into their own genome pool.
All Tyranid organisms are Synaptic (psychically-reactive), and each Tyranid creature within a Hive Fleet shares in a communal Hive Mind, granting the ability for trillions of beings to communicate and organize instantaneously on a staggering scale.
From beyond the very edge of the galaxy come the Tyranid Hive Fleets – tendril-like swarms bent on consuming all life. Guided by a terrible communal intelligence known as the Hive Mind, the Tyranids exist solely to strip the universe bare to feed their endless appetites. In a matter of weeks, Hive Fleets can strip whole systems of life. They are without number, without fear and without mercy.
A Tyranid Hive Fleet is controlled by a terrible communal intelligence known as the Hive Mind. The largest of the hive ships serves as the repository of this mind, but it spreads to every Tyranid. Individuality does not exist among the Tyranids. All serve the hungers of the hive.