The Tyranids are a race of terrifying aliens from another galaxy. Their mission is to consume all biological and genetic material. This allows them to evolve and reproduce. There is no explanation for this drive to consume, there is no reasoning with it. The Tyranids exist in a collective. They swarm through the galaxy in Hive Fleets over over a billion creatures, directed by a Hive Mind. When they pick a planet to consume, they systematically attack, overtake and consume her inhabitants. They are without a doubt the greatest threat to the Galaxy.