The Halo Array, sometimes known simply as Halo or the Array, is a network of 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 first array of twelve Halos was constructed by order of Master Builder Faber circa 98,445 BCE with the intent of being an ultimate weapon, one that would secure the preeminence of the Builder rate in Forerunner society. Later, the weapons would be refined further and a new array of six smaller and significantly more lethal installations was constructed; the newer Halos are known as the "neoteric" rings. Eventually, eleven were destroyed and/or lost while the remaining seven were used by the Forerunners as a last resort when combating the parasitic Flood. All sentient beings in the galaxy were killed, robbing the Flood 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 Flood 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.