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Installation 05, or Delta Halo as it is called by humans, is one of the seven massive Halo fortress worlds built by the Forerunner.
Details
Planet of Orbit: Substance
Monitor: 2401 Penitent Tangent
Status: Intact; on standby mode
History
It is assumed that Installation 05 was built around the same time as Installation 04, around 98,665 BC. It was activated shortly thereafter by the Forerunner, killing all sentient life in the galaxy and, presumably, destroying the Flood as well, with the exception of those specimens kept in the Halos for research. As of 2552, it had a record of 1.2 trillion simulated firings and one actual.
Flood Disaster
There was some occurrence on Installation 05 that caused the Flood to be released. It is possible that another alien race stumbled on the Installation and accidentally opened its Flood Containment Facilities, but whatever happened, the Flood were released. They promptly grew out of control around the area of the Installation's Library. The Flood were kept contained by a Sentinel Wall built around the Library, but 2401 Penitent Tangent was captured. Without it, the Sentinels of the Halo lacked a controlling intelligence and so many of the structures around Installation 05 fell into disrepair, and most of the Installation has come to look like a jungle filled with ruins. The central Flood Brain Form became so massive that it reached sentience, calling itself the Gravemind. Gravemind, with 2401 Penitent Tangent as its slave, managed to keep the fighting between Flood and Sentinels a stalemate. Even with the addition of Forerunner Sentinel Majors and Enforcers, the Flood stayed loose, though contained, inside the Sentinel wall as the structures of the Installation slowly degraded. The Gravemind itself resided in the chamber directly below where the Index was stored in the center of the Library.
Discovery
In 2552, the High Prophet of Regret, a Covenant hierarch, discovered the location of Installation 05 during The First Battle of Earth. He made a slipspace jump from Earth to the Halo, followed closely by the human ship UNSC In Amber Clad.
When they arrived, a human called the Master Chief tracked down and killed Regret, just as the Covenant city of High Charity arrived with hundreds or thousands of ships. Covenant landed on the Installation and foolishly deactivated the Sentinel Wall. Flood were able to infect much of the Human and Covenant forces, even taking control of the In Amber Clad and High Charity. The Covenant Arbiter retrieved the Index, believing it to be necessary to begin the Great Journey, but he was almost killed by the Brute Tartarus. He fell into the chamber of the Gravemind, where he met the Master Chief, 2401 Penitent Tangent, a Prophet form of Regret, and the Gravemind. There they "edified" him of the true nature of the Installation. Meanwhile, Tartarus brought the Index to the Halo's Control Room. He activated the station, but was killed before it could fire. The Index was removed from the chamber, and Installation 05 sent a signal to the other Halos putting them on standby mode, ready for remote activation through The Ark.
When Installation 05 was last seen, Flood were loose all over it, as well as in High Charity and the surrounding fleet. The Flood appear to be in control of the Installation, as the Civil War of the Covenant has caused fighting between the remaining Covenant forces on the ring.