The Halo Array is a network of seven ring-shaped artificial worlds created by the Forerunners in order to kill all sentient life within range of the array, virtually the entire galaxy. Rather than a weapon of war, these were used as a last resort against the Flood, a parasitic extra-galactic species that threatened to infest every sentient lifeform in the galaxy.
By activating the rings, the Forerunners denied the Flood access to these lifeforms, effectively starving the species to virtual extinction, though it clung to life in a number of Forerunner facilities, including some Halo installations.
