Halo Array (Earth-2915)

"Halo doesn't kill only xenomorphs, it kills their food! Humans, Covenant, whatever—we're all equally edible! The only way to stop the xenomorphs is to starve them to death. And that's exactly what Halo is designed to do: wipe the galaxy clean of all sentient life!"

- Cortana

The Halo Array, sometimes known simply as Halo or the Array, is a network of seven colossal ring-shaped superweapons individually known as Halos or Halo rings, constructed by the forerunners. The Halos were also referred to by the Covenant as the Sacred Rings, and labeled as Installations or infrequently referred to as Fortress Worlds by the forerunners and their monitors.

The seven Halo rings were used by the forerunners as a last resort when combating the endoparasitic xenomorphs. All sentient beings in the galaxy were killed, robbing the xemomorphs of hosts and starving it to death.

In addition to serving as weapons of last resort, Halo installations are also research facilities dedicated to containing and studying many different kinds of life forms. The life forms that were hand-selected by the Librarian included marine life, fauna, and land vertebrates. With the Master Builder's permission, the Librarian would research each animal to detect if their bodies reacted to xenomorph infection. The research on the animals would be more deeply analyzed if they did become infected. The Librarian's studies also helped determine if the rings could be used to repopulate the galaxy as part of the Conservation Measure.

Source

 * https://www.halopedia.org/Halo_Array.