From Halopedia, the Halo wiki
The Repository is an underground Forerunner facility located on Installation 07. During the Installation 07 conflict, the Banished constructed their headquarters, the House of Reckoning, above this facility.
On December 13, 2559, when Cortana sacrificed herself to destroy the Silent Auditorium and deny Atriox control of Zeta Halo, a large section of the Ring broke off, taking with it several Forerunner facilities including the Repository. Months after her death, fragments of Cortana's data lingered in and around the Repository, manifesting themselves as ghostly echos of various memories.
At some point during the Installation 07 conflict, at least three UNSC marines entered the Repository and battled the Aggressor Sentinels guarding it. The outcome of this battle is unknown, but the bodies of the humans and Sentinels who fell in this fight were later discovered by John-117. Also at some point during the war, the Banished and the Harbinger's Skimmers occupied the Repository and allied themselves with (or took control of) the facility's Sentinels.
On May 28, 2560, shortly after the Master Chief and the Weapon infiltrated the Command Spire and shut down the Spire network, Blademaster Jega 'Rdomnai hijacked Pelican Echo 216 and kidnapped the pilot, Fernando Esparza. Determined to rescue Esparza, the Chief had the Weapon open a portal that would take them as close as possible to wherever Jega and his prisoner were heading. The portal sent the Chief through a data cluster where he experienced a dream-like vision of Cortana's memories. Upon waking up, the Chief found himself inside of the Repository. The Chief and the Weapon explored the underground facility and found a terminal, which the Weapon used to determine that they had arrived near a Banished fortress. War Chief Escharum then contacted them and demanded that the Spartan face him at the House of Reckoning. Escharum threatened to continue torturing Esparza until the Master Chief accepted his challenge.
As the Chief and the Weapon made their way to the surface, they came across other data clusters containing more of Cortana's memories. The memories showed Cortana's increasingly ruthless tactics during the Created conflict, culminating in the destruction of the Jiralhanae homeworld, Doisac. Upon witnessing these memories, the Weapon realized that she was actually an exact copy of Cortana created by Catherine Halsey. The Master Chief confirmed this and explained it was the only way to defeat Cortana. Terrified of becoming exactly like her predecessor, the Weapon asked the Chief to delete her. However, the Chief refused and said they needed to work together to rescue Esparza. The Chief and the Weapon then left the Repository and returned to the surface.
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