In the event of a Flood outbreak, one of the seven installations could be triggered (in turn triggering the remaining Halos) to annihilate all sentient life within a radius of 25,000 light years. The purpose of Installation 04, along with its brethren, was to preclude and thwart the parasite forms known as the Flood from consuming all sentient life in the galaxy by destroying all sentient life to prevent eventual consumption. Installation 04 serves as the main setting of Halo: Combat Evolved, is briefly seen in the beginning of Halo 2 (albeit destroyed), and briefly at the end of Halo: Reach.

A second replacement was then built, this one being completed successfully. The Ark immediately began constructing a replacement, but that too was destroyed a few months later. The installation was destroyed as the result of the crashed UNSC Pillar of Autumn's reactors detonating on September 23, 2552. It was located over the gas giant of Threshold in the Soell system, and was the closest of the Halo rings to human territory. Installation 04, known to the UNSC as Alpha Halo or simply Halo, and to the Covenant as the First Holy Ring or First Sacred Ring, was the fourth of the seven ring-shaped superweapons that was constructed by the Forerunner Ecumene as part of the Halo Array. ' Whoever controls Halo controls the fate of the universe.' - Captain Jacob Keyes. Soell system, in orbit around Threshold, Orion Arm, Milky Way For other uses, see Installation 04 (disambiguation) Installation 04

Battle of installation 04