From Halopedia, the Halo wiki
Librarian's Rest was a location inside the Forerunner shield world Requiem. The Forerunner structure was situated near the area codenamed "Sniper Alley". It hosted a shrine inholding the Librarian's imprint and later became a major emplacement in the conflict opposing the United Nations Space Command to the reformed Covenant.
In July 2557, during the Battle of Requiem, John-117 found the Forerunner structure which contained her imprint. Inside the shrine, the Librarian revealed to him the truth about the ancient humans and asked him to stop the Ur-Didact. She provided him an immunity to the Composer, but was unable to tell him more as her husband had found them. The Chief then left the structure.
In February 2558, during the Requiem Campaign, both the UNSC and the Covenant sought the Librarian's imprint, but Jul 'Mdama's forces were the first to find "Librarian's Rest." However, the shrine's activation required a Reclaimer. Via a Forerunner artifact which was brought onboard the UNSC Infinity by its crew, the Covenant managed to abduct Dr. Henry Glassman and forced him to work on opening the shrine. They thought the Librarian would gift them incredible bounties spread throughout the galaxy, such as weapons and ships. Despite lacking adequate equipment, Glassman managed to activate the shrine. When all the Sangheili were captivated by it, he escaped with the help of Spartan Gabriel Thorne who had been captured and taken to "Librarian's Rest." However, Glassman only managed to activate the shrine's protective shield rather than the ability to enter it.
Later, after an attack on the Infinity, the Covenant abducted Dr. Catherine Halsey to help them unlock the structure. At "Librarian's Rest," Dr. Halsey deactivated the protective shield with her data pad. She then quickly ran into the beam of light before 'Mdama. Inside the shrine, Halsey met the Librarian, who offered her the two halves of the Janus Key, an artifact capable of locating every pieces of Forerunner technology in the galaxy. She asked her to bring it to the Absolute Record to help mankind. Halsey was subsequently expelled from the shrine.
Once Dr. Halsey was expelled from the shrine, 'Mdama took the red half of the key, while the doctor tossed the blue half to Gabriel Thorne, who had just arrived. Commander Palmer and the rest of Fireteam Majestic then arrived and clashed with Sangheili Storm and Promethean Knights. 'Mdama was able to escape with a wounded Dr. Halsey thanks to a Knight who translocated them away, resulting in the failure of the Spartans' mission. As all the enemies were either dead or gone, the Spartans-IVs left the "Librarian's Rest." Later, as the Librarian had predicted, Requiem was set on a collision course with its sun, which ended in the destruction of the shield world, but not the Librarian's imprint as she had been transferred to the Absolute Record with the help of Fireteam Crimson.
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