Overview
One of the few battles of the 9th Age of Reclamation in which all four of the main factions took part. The Covenant and UNSC fought through the Quarantine Zone in search of the Index, known to the Covenant as the Sacred Icon. Both parties had to battle their way through legions of Flood and the Forerunner machines designed to halt the parasite outbreak.
The Battle
Background
Not much is known of how the battle of the Quarantine Zone began, as it is rooted in ancient history. The Battle began after an unknown event approximately 100,000 when Flood under containment on Installation 05 were released. Under normal circumstances Sentinel guards would have been able to keep the parasite under control. Unfortunatly, the Flood had access to hosts, and were able to hold off the constructs.
After building enough biomass, a local Proto-Gravemind reached critical mass, allowing the formation of a sentient, highly aware for of flood known as the Gravemind. Now coordinated, the Flood posed a serious threat. The battle soon spiraled out of control, and Sentinel Constructor Factorys were built to construct more powerful constructs, such as Sentilel Majors and Enforcers. An enourmous wall was built around the Installation's Library, which was housing the Gravemind[1]. The area within the boundries of the wall were starved of heat and light in order to inhibate Flood growth, and thus the Quarantine Zone was established.
Over the centuries, the Gravemind's reach spread throughout the bouls of the Installation[2]. The Monitor of the installation, 2401 Pentitent Tangent was captured by the Gravemind, giving him access to the Teleportation Grid[3], and causing the structures on the Installation to fall into disrepair.
Covenant Intervention
The Arbiter of the Covenant was sent to the surface of Installation 05 to retrieve the Installation's Index. Dropped directly onto the Sentinel Wall by a Phantom transport, the Arbiter fought mostly alone, only aided by a small number of the original landing party (several Grunts and Jackals), through numerous Forerunner constructs to reach the control center for the containment shields. By lowering these shields, the Covenant soldiers learned that Human Marines had made it through the wall only to be swiftly infected by the Flood. More Covenant warriors soon entered the Quarantine Zone, unfortunately exacerbating the Flood outbreak.
This breach of containment led to the rapid growth in the Flood's numbers as they assimilated both humans, from the UNSC In Amber Clad, and Covenant warriors. With these new hosts, the Flood gained access to more sophisticated equipment, weapons and combat vehicles, magnifying the danger the outbreak presented immeasurably. The Flood showed a bit of a tactical genius by destroying one of the Forerunner Sentinel Constructor ships. It ceased to float above the Quarantine Zone and produce countless Sentinels and Enforcers.
Despite this new and surprising defense, Covenant and Human forces were able to battle their way through the legions of Flood and Sentinel constructs. Elements of both parties eventually reached the Gondola entrance points to the Library of Delta Halo. Commander Miranda Keyes, Sgt. Johnson and a number of Marines entered the Library first. They were quickly followed by the Arbiter and a squad of Elite troops. Rtas 'Vadumee stayed behind to defend the Gondola station, presumably he did not do so alone.
The Library
Human forces had gained access to the Library first, but only by a matter of minutes. Upon arrival within the Library it was clear that the humans had faced stiff opposition, all but a few of their forces had died combatting the Flood and Sentinels already within its walls.
The Arbiter found Miranda Keyes and Sergeant Johnson within the Index Chamber having retrieved the Index seconds before his arrival. Sergeant Johnson was ready, armed with a Battle Rifle, along with Miranda Keyes who was reaching for the Index holding on to one of the Gravemind's tentacles. Miranda secured the Index and drew two SMGs. Johnson called for his marines, but received no answer and armed his weapon. The Arbiter proved his worth as a warrior once again, incapacitating both Johnson and Keyes in a matter of moments. The arrival of Tartarus within the Chamber signified the beginning of the Civil War of the Covenant as he attacked the Arbiter, sending him plummeting to his doom deep within the Library, towards the Gravemind. The battle itself was a religious victory for the Covenant, but an overwhelming tactical victory for the Flood. The Flood succeeded in not only escaping Installation 05 but gained the means to spread to other worlds. The UNSC however failed to recover the Index and were almost completely killed or assimilated into the flood. This marks one of the few UNSC defeats in the Halo games
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There is a downed Pelican near the gondola access. It is believed that Miranda Keyes, Sergeant Johnson and the other Marines came in that Pelican after the shield went down.
Possible Human/Sentinel Alliance
There is supporting evidence to entail and suggest that the Marines were never attacked by Sentinels but were in fact assisted by them. This is not out of the question because, like on Installation 04, the Sentinals might've identified the Marines as Reclaimers rather not on Onyx where the Sentinels branded the Humans as an "Aboriginal Subspecies." This would also explain their open hostility to the Covenant who were enemies of the Humans. For starters, the Arbiter encounters an enormous and massive number of Human Combat Forms inside the Sentinel Wall immediately after he deactivates the shield. This suggests that the humans had been deployed much earlier which should've been impossible. First of all, the marines should not have been able to deploy immediately after the barrier went down because In Amber Clad would've first had to slip past the Covenant forces that were moving in to secure the inner wall. However, if the Sentinals had chosen to assist the marines, they would've deactivated a small portion of the barrier to let them in. Also, in the area where the UNSC Pelican flies over head, there are a number of Sentinels and even an Enforcer present. None of which seem to show any hostility towards the Pelican. Another factor is simply that considering how much trouble Covenant Elites had getting inside the Library, it doesn't seem likely that the marines would've been able to get past the Flood and the Sentinals. However, if the Sentinals had been helping them, then it's much more likely that they would've succeeded.