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After its destruction at the end of the Battle of Installation 04, the Ark started to construct a replacement for Installation 04. The Covenant reverentially described Installation 04 as either the First Holy Ring or the First Sacred Ring. It was located in the Soell system in the Orion Arm and was the closest Halo ring to the human sphere. Installation 04, also known as Alpha Halo, or simply Halo, was one of seven ring-shaped superweapons that comprised the Forerunners' Halo Array.
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Soell system, in orbit around Threshold, Orion Arm, Milky Way
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