In the distant past, a powerful race called the Forerunners fought an alien parasite known as the Flood. The Flood, which spread through infestation of sentient life, overran much of the Milky Way Galaxy. One of the races affected was humanity, who came into conflict with the Forerunners as a result. Exhausted by their war and after having explored all other options, the Forerunners conceived a weapon of last resort to combat the Flood, called the Halo Array. Using an installation known as the Ark, they built large ring-shaped megastructures known as Halos. The Halo Array, when activated, would destroy all sentient life in the galaxy – depriving the Flood of their food, after which samples of the lost lifeforms would be reseeded. Delaying as long as they could, the Forerunners activated the rings and disappeared.