Four countless of years the Forerunners ruled the galaxy with iron fist, dominating "lesser" species and in constant odds against Humanity's Imperium of Man and it's Enclave (alongside the San'Shyuum), and the minor treath of the Protoss. They managed to win every time duo to their quintillion-sized fleets and perceverance, which ultimately resulted into the near destruction of Humanity. All happened as they planned until the arrival of a much more dangerous treath, the Flood, a parasictic alien race that feed upon both sentient and non-sentient life to survive. The Forerunners fought all they could, but their enemy was unstoppable.