The Greater Ark was a Forerunner facility similar to the Lesser Ark seen at the end of the Human-Covenant War, except far larger. While the Lesser Ark produced the 10,000 kilometer diameter Neoteric Halos such as Alpha Halo and Delta Halo, the Greater Ark produced 30,000 kilometer diameter halos for the Senescent Array. The original design of the halos was to be used as a more localized defense from the Flood, and the galaxy-wide Neoteric array was only devised much later.
After the destruction of the Forerunner capital world of Maethrillian towards the end of the Forerunner-Flood War, the Greater Ark became the home of Forerunner governance. This eventually came to an end as well when the Greater Ark, Omega Halo, and all remaining Forerunner forces were attacked during the Battle of the Greater Ark. Nearly all Forerunner forces were destroyed, including the Greater Ark itself. Only a handful of Forerunners managed to escape the battle and fire the Neoteric Halo Array from the Lesser Ark.