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 mentality of the Tyranid approach to warfare can be described with the phrase "quantity has a quality all its own". From the "lowly" Ripper, to the deadly Hive Tyrant and beyond, the signature of the Tyranid force is that they overwhelm their foes with sheer numbers, reproducing massive numbers of highly virulent organisms in record time from the biochemical soup that they derive from the biospheres of the worlds that they consume. The components of a Tyranid Hive Fleet travel almost exclusively in large groups known as swarms that possess specialized biomechanical creatures for destroying and consuming a wide variety of prey life forms. Tyranids have evolved sophisticated methods for facilitating genetic transfer across species boundaries. As a result, a significant goal of any Tyranid invasion is acquisition of useful new biological traits from other lifeforms. These are used by the Hive Mind to enhance the Tyranids' effectiveness in consuming new worlds to gain more of the necessary organic raw materials for further reproduction. All Tyranids are reproduced by a single, highly intelligent female bioform known as a Norn-Queen. A Hive Fleet's Norn-Queens are the most important Tyranids within the fleet, for if they are injured or killed the Tyranids cannot reproduce their numbers from the captured bio-mass. As a result, Norn-Queens can be found only at the heart of the largest and most-well defended Tyranid Hive Ships.
The Tyranids encountered the Empire of the Combine Race years before the First Multiverse War and became members of the Coalition of Independent States. With the help of the Orks, they managed to evolve faster and become a powerful enemy against the Combine and the Alliance of Nations. They later joined the Hegemony of Independent States in support of a stronger military power.
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 the Empire of the Combine Race 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.