Tyranid

Origins
The origins of the Tyranid are unknown; though it is a fact that they are not of the Mily Way Galaxy, having arrived by traversing the intergalactic void, their home planet and purpose of their genocidal rampage is unknown. Even their name is one given to them by the Imperium, as Tyran was the first world they were encountered on, and the first world they devoured. The Astronomican is what drew the Tyranid Hive Fleet to the galaxy, though it is postulated that the Tyranid have been devouring galaxies since the beginning of time.

The first recorded encounter between the Imperium and the Tyranid was in 741.M41, when the Hive Fleet later known as Behemoth invaded the Eastern Fringe, ultimately destroying the planet Tyran. There is evidence however that the Tyranid have been slowly invading and probing the galaxy for much longer, with certain species (such as the Catachan Devil and Fenrisian Kraken) postulated to be genetic descendents of the invading Tyranid fleets.

Culture
There is no indications that the Tyranid have any sort of culture or philosophy; instead, the Tyranid operate on a hivemind principle, where each individual mind is controlled by a collective force of all minds. Though this makes them incredibly powerful as a collective whole, it is postulated that this is the ultimate weakness of the Tyranid; find a way to disrupt the psychic link, and you disrupt the Tyranid. There is also some theories that postulate this overwhelming psychic presence is either an indication of an Old One or a psychic monster, and that the Tyranid are simply puppets of this overwhelming psychic being.

Military
The Tyranid fleet exists solely to carry out the will of the swarm. Because of this, the Tyranid can create units of specialization, but more often than not, create the same standard units for the same standard purpose - killing. The Tyranid doesn't have a classical structure commonplace to military constructs, as no individual unit, no matter rank or position, controls the hivemind more than any other unit.

Allegiance
The Tyranid have no ability to comprehend allegiance, as their only loyalty is to the Hivemind which controls their actions.