 " Halos " are massive ringworlds , ranging from thousands to tens of thousands of kilometers in diameter , scattered across the galaxy . These rings were constructed thousands of years ago by a race known as the Forerunners as weapons of last resort against the parasitic alien species known as the Flood . When activated , the Halos would destroy all sentient life in the galaxy , depriving the Flood of its food . The Forerunners disappeared after they activated the rings . In Halo : Combat Evolved , whilst fleeing the Covenant , the UNSC ship Pillar of Autumn stumbled upon one of these ringworlds , Installation 04 . The humans manage to destroy the ring , stopping the Flood ; the Covenant , unaware of the destructive nature of the rings , attempt to fire another ring , Installation 05 , during Halo 2 in order to fulfill their religious prophecy . One race in the Covenant , the Elites , learn the truth about the rings , and join forces with humanity in order to stop the firing of the ring . Though they are successful , the unexpected shutdown of the installation triggers a fail @-@ safe protocol , priming all the rings for firing from one location , referred to as the Ark . Still oblivious to the true nature of the rings , the Covenant High Prophet of Truth and the remaining loyalist Covenant proceed to head to Earth , where they believe the Ark is buried . 
