Installation 04 (Earth-2042)

"Whoever controls Halo controls the fate of the universe."

- Captain Jacob Keyes

Installation 04, also known as Alpha Halo, or simply Halo, was one of seven ring-shaped superweapons that comprised the Forerunners' Halo Array. It was located in the Threshold system in the Inner Rim and was the closest Halo ring to the Galactic Republic space.

The Covenant reverentially described Installation 04 as either the First Holy Ring or the First Sacred Ring. After its destruction at the end of the Battle of Installation 04, the Ark started to construct a replacement for Installation 04. After the replacement was destroyed, the Ark constructed yet another replacement.

Source

 * https://www.halopedia.org/Installation_04.