Tyranids

“There is a cancer eating at the Imperium. With each decade it advances deeper, leaving drained, dead worlds in its wake. This horror, this abomination, has thought and purpose which functions on an unimaginable, galactic scale and all we can do is try to stop the swarms of bio-engineered monsters it unleashes upon us by instinct. We have given the horror a name to salve our fears; we call it the Tyranid race, but if it is aware of us at all it must know us only as Prey.”
— Inquisitor Czevak at the Conclave of Har

The Tyranids are a species of terrible xenos that is actually a space-faring ecosystem dedicated solely to its own propagation and evolutionary advancement. The Tyranids collectively form a monstrous superorganism that travels the universe in their great Hive Fleets of biomechanical Hive Ships, systematically consuming all other biomatter to enable its own rapid evolution and reproduction. 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.

The Tyranids are actually an extragalactic species, born somewhere beyond the void that separates the galaxies of the Local Group. Tyranid Hive Fleets travel through space by using gigantic, organisms genetically engineered to travel through a vacuum known as Hive Ships, which move in groups which can be likened to very large locust swarms. Typically, Tyranid Hive Fleets move in ad hoc formations known as Tendrils, migrating to nearby inhabited planets after consuming all the biological and organic material on a recently invaded planet and breeding, often reproducing many times their original number. Hive Fleets travel with superluminal speed, though they never enter the Warp. Instead, by using the specialized psychic power of the a selectively evolved Hive Ship, they manipulate the gravity fields of star system to achieve faster-than-light travel. 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. Given that each Tyranid Hive Fleet has approached the Milky Way from a different direction, this may imply that the Tyranids have consumed a disturbingly large number of nearby galaxies.
Tyranid Hive Fleets consist of millions of biomechanical craft, each serving as host to an untold number of symbiotic organisms. These creatures evolve and spawn from a variety of geno-organisms in the ships’ reproductive chambers. All these creatures are born to service the ship, and the ship in turn exists only to service the needs of the Hive Fleet.

Whereas other armies like those of the Imperium and the Tau fight for conquest or self-defence, and the Forces of Chaos and the Ork clans battle merely for the chance to wreak violence and havoc, the Tyranid Hive Fleets are driven by the will of the Hive Mind, which itself is motivated by the most ineradicable instinct inherent to all lifeforms – reproduction, and through reproduction and the acquisition of new genetic traits, evolution to a better adapted form.

A fully mobilized Tyranid Hive Fleet strikes shock, awe, and terror into even the most stalwart of humanity’s defenders. Even when the xenos threat is eliminated (at great cost), it will not be long before another Tyranid migration emerges to take advantage of the situation. This is what makes the Tyranid species such a threat: it is an evolved metapredator capable of out-producing, out-consuming, and out-lasting all other species it encounters, having done so on a disquietingly unknowable number of prior occasions across a vast swathe of intergalactic space.

Tyranids

Rogue Trader: Season One bscarbeau