The Halo Array was a group of seven ring - shaped superweapons constructed by the Forerunners to destroy the flood. The Seven installations were ringworlds which, when used all at once, would kill all intelligent life within the entire milky-way galaxy.
The Halo Array was constructed by and controlled from Installation 00, also known as the Ark, located outside the milky-way, contained the equipment to restore life in the galaxy and to fire all the Halo Array at once. They where built by the Forerunners to kill the flood by killing it's food, and all intelligent life forms in the galaxy, thereby starving the parasite to death. It was fired around 100,000 years ago, and succeeded in its task, but a few flood were kept in stasis on a few of the Halos, so they could be studied. These Flood were accidentally revived by the covenant on installation 04. Installation 05 had an isolated outbreak, which resulted in the formation of a Gravemind.