The Tyranids. Very little is known about them, they first appeared when a space marine ship of the death-watch went to investigate the planet tyran witch hadn’t been heard from for some time.
The Death-Watch contingent had found that everything had been completely whipped out even its core had been somehow removed thereby getting the name of Tyranids. They are supposedly the first Tyranid hive Fleet to have entered our galaxy but certainly weren’t the last, they have massive hive fleets and are virtually with-ought number. They are truly powerful and large creatures with a drastically high evolution rate and can quickly adapt to overcome a form of weaponry or evolve a biological counterfeit. They come in very different sizes and are very powerful with razor sharp claws or bio-weapons that evolve onto them.