From Halopedia, the Halo wiki
Reformation is the process by which a Halo Ring repairs itself.
Whenever a Halo Ring suffers damage, its Sentinels (assuming there are any left) almost immediately begin to assess the damage and initiate repairs. However, depending on the severity of the damage, this natural repair process can potentially take millions of years to complete. The Reformation is meant to greatly accelerate the Ring's restoration. During the Reformation, multiple Reformation Spires emerge from the surface of the Ring in close proximity to the damaged areas. Once fully activated, the Spires create raw materials which are then sent out to repair the Ring. The Spires are controlled by a Command Spire, which itself is controlled by the Nexus, a facility located directly beneath the Command Spire.
During the Installation 07 conflict, the Banished and the Harbinger sought to utilize the Reformation to restore the Silent Auditorium, which had been destroyed when Cortana sacrificed herself to stop Atriox from gaining control of Zeta Halo. On May 28, 2560, the Harbinger kidnapped Despondent Pyre, the Monitor of Zeta Halo, and extracted her encephalon. Using the Forerunner routines contained within Despondent Pyre's encephalon, the Harbinger constructed a protocol which she used to summon dozens of Reformation Spires across the fragmented section of Zeta Halo. The Spires still required an initialization sequence to become fully activated, so the Harbinger traveled to the Command Spire to personally initiate the Reformation.
Master Chief John-117 and his AI companion, The Weapon, entered the Reformation Spire nearest to the Conservatory, where they had first encountered the Harbinger. After learning of the Spire's purpose, the Spartan-II ordered the Weapon to shut it down. In response, Zeta Halo's Sub-Monitor, Adjutant Resolution, who was unaware of the Harbinger's true intentions, deemed the pair a threat to the Reformation and attacked them. After the Chief defeated Adjutant Resolution, the Weapon discovered that multiple Spires were being deployed. She also discovered that the Harbinger was attempting to activate the Spire network from a remote location, but that the Spires were resisting the latter's efforts because the protocol used to summon them was only partially correct. The Weapon managed to remove the Spire she and the Chief were standing on from the network, as well as trace the source of the activation attempts to the Command Spire. However, this not only caused their Spire to start sinking back into the ground, it also alerted the Harbinger to the Weapon's existence.
After the Spartan Killers Hyperius and Tovarus failed to kill the Master Chief, the Harbinger instructed Escharum to have his forces protect the Command Spire while the Reformation was underway. She also sought to learn more about the Spartan's ally. As part of this plan, the Harbinger altered the Forerunner sequence needed to open the Command Spire's door, thereby making it impossible to open. The sabotage forced the Chief and the Weapon to gather data from a network of Forerunner communication beacons in order to reconstruct a pure version of the sequence. During this time, the Harbinger spied on the Weapon and determined she could utilize the UNSC AI's routines in conjunction with those from Despondent Pyre's encephalon to break through the Spire network's resistance.
While collecting data from the beacons, the Weapon sensed the Harbinger's presence within the beacon network and eventually realized the latter's intentions. Since the Spires still hadn't been activated yet, the Master Chief initially wanted to abort their current plan, effectively leaving them and the Harbinger in a stalemate. However, the Weapon believed that the Harbinger would eventually find a way to activate the Spires on her own, so they had no choice but to finish gathering the data needed to enter the Command Spire.
Later, upon reaching the Nexus, the Weapon accessed the facility's systems in order to activate a gravity lift that would take the Chief and her into the Command Spire itself. The Harbinger sprung her trap and hacked the Weapon, intent on using her to start the Reformation. The Weapon, in turn, used her link to the Harbinger to try to steal Despondent Pyre's encephalon. Despite the Master Chief's attempt to delete the Weapon - an effort which the Weapon herself stopped - both the Harbinger and the Weapon succeeded in their respective goals. This caused friction between the Chief and the Weapon, with the Chief being angry that the Weapon had stopped him from deleting her and had thus allowed the Reformation to begin, while the Weapon - who believed the Harbinger would have found a way to start the Reformation regardless - was angry that the Chief had tried to delete her in the first place rather than trust her.
With the Reformation underway, the Spires began forging building blocks which were sent out to repair the Silent Auditorium. The Chief fought his way to the Command Spire's observation platform, where the Weapon used Despondent Pyre's encephalon to shut down all the Spires and end the Reformation. By then, however, enough of the Auditorium had been restored, such that the Harbinger could now use it to locate and free her people.
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