Installation 04 'Alpha Halo'

Installation 04 was built by Installation 00's Foundry before 100,000 BCE. When the IsoDidact activated the Array at the Forerunner-Flood war's conclusion, ending the centuries-long conflict, Installation 04 was the first to fire, followed shortly after by the other six Halo rings. Installation 04 lay dormant for a hundred thousand years, with 343 Guilty Spark and his Sentinels ready to keep the Flood confined in the Flood containment facilities and keeping the infrastructure of the Halo in perfect working order.

After eons of uneventful existence on Installation 04, 343 Guilty Spark eventually found himself bored. For his own amusement, Spark began an experiment in which he evacuated entire sections of the Installation into space; according to Spark, these were experiments to measure the geologic effects of exposure to the vacuum of space and the eventual biological recovery of the evacuated sections, in order to gain insight into emergency response scenarios.

Around 40,000 BCE, an unidentified alien vessel crash landed in Sector 1215. Sometime prior to the crash the ship started broadcasting a distress signal. After attempting to translate it, 343 Guilty Spark blocked the signal, as no communication revealing the location of any of the Halos was allowed. After no lifeforms exited for several days, Spark had a large sarcophagus constructed around the vessel.

Between the activation of the Array and the arrival of the UNSC and the Covenant, a number of space-faring species stumbled across the installation. Although they reportedly explored the facility and recorded their findings, all of these species eventually left without breaking containment.

The Covenant discovered the location of Installation 04 after the Covenant Fleet of Particular Justice followed the UNSC Pillar of Autumn, which was fleeing from the Fall of Reach through slipspace. The Halos are deeply ingrained into the Covenant's religious dogma, but prior to this discovery they had never actually found one. Due to the Covenant's faster slipspace capabilities, they arrived before the Pillar of Autumn.

The Pillar of Autumn arrived around September 19, 2552 and was promptly shot down by the warding Covenant ships. Shortly thereafter, the Covenant ground forces on Installation 04 accidentally unleashed the Flood from captivity. Humans, Flood, and the Covenant fought massive battles against one another as the Forerunner constructs tried to control the outbreak by activating Halo. The ring's monitor, 343 Guilty Spark, tried to convince the Master Chief to activate Halo. Not knowing the truth of what it did, he attempted to light the ring but was stopped by his AI friend and companion Cortana who was in the Core. She informed the Master Chief of Halo's true purpose and stashed the Activation Index in her memory banks. The Monitor betrayed the two and tried to kill the Master Chief and retrieve Cortana, but the two escaped and started plotting against him.

In the conclusion of the battle, John-117 detonated the Pillar of Autumn's fusion reactors in order to destroy the ring. The explosion critically damaged the superstructure below it; the resulting shockwave and damage to the ring's systems destabilized the ring, shattering it into pieces.

Only a handful of humans and Covenant survived the battle. 343 Guilty Spark floated through space to a gas mining platform over Threshold, where he stumbled upon a Covenant expeditionary force. Inspired by his words, they began a heretical rebellion against the greater Covenant. Installation 04's destruction is seen as the first step in the chain of events that led to the Great Schism. The Heretics were quelled by Arbiter Thel 'Vadam who silenced their leader, but the seeds of a rebellion were planted. The Monitor was then captured by the loyal Covenant and later defected to the humans' side until he was destroyed on the replacement Alpha Halo by SPARTAN-117 with the help of Sergeant Johnson.
Installation 04 was the first Halo installation encountered by humanity, as well as first to be destroyed. Its destruction triggered the activation of a portal leading to Shield World 006 within Onyx, and also caused the Ark to start constructing a replacement ring.

There is now a large field of debris where Installation 04 was, though a few large chunks of the ring remain. Some smaller pieces of the Installation 04 have landed on Basis, while others have burned up in Threshold's atmosphere upon entry. The UNSC's peace treaty with the Covenant forbids either party from landing on the remains of Installation 04.

When the Pillar of Autumn's reactors went critical, Installation 04 attempted to activate a safety measure and make a slipspace jump. However, this self-preservation protocol was only partly successful, and only a small portion of the ring managed to enter slipspace before the Autumn's reactors overloaded and destroyed the ring. The surviving fragment of the Halo reentered normal space in the vicinity of a red giant star some distance away from the Orrichon system and was caught in the giant star's orbit. This fragment was home to a unique element apparently created by the heat of the ring's explosion and not found elsewhere in the universe. The element was later refined and used in a bioweapon unleashed in a terrorist attack on Sedra City.

Following the return of the survivors from the battle, the Office of Naval Intelligence learned of the Flood lifeform. Section III sought to establish a research laboratory in order to control the Flood parasite and possibly weaponize it against the Covenant. A science team headed by Major John Smith was authorized to take command of the prison vessel Mona Lisa. After bringing a group of Covenant prisoners aboard, they jumped to the wreckage of Alpha Halo in the Soell system. Some Flood specimens had survived the ringworld's destruction and were recovered to be experimented on both human and Covenant prisoners. However, the Flood soon broke quarantine and infected the ship. ONI later dispatched a prowler, the UNSC Red Horse, to assess the status of the Mona Lisa under the pretenses of investigating Halo's debris field. On October 25, 2552, the prowler was forced to destroy the overrun vessel with a Shiva-class nuclear missile.

Shortly after the destruction of the Halo, Installation 00 began to automatically construct a new Halo to replace Installation 04. This exact copy of the first Halo ring was nearing completion only a few months after the first ring was destroyed, but was also subsequently destroyed when it was fired before it was ready by the John-117 and Cortana. 343 Guilty Spark attempted to prevent this, but was finally destroyed by the Master Chief with the help of Sergeant Johnson in this new ring's Control Room.