From Halopedia, the Halo wiki
Driven by her own geas from the Precursors to fix the path and set right what the Forerunners did wrong, the Librarian located the specimens left behind by two deceased Precursors while in Path Kethona searching out the origins of the Flood. Returning home with the specimens, the Librarian kept her discovery out of her report to the Ecumene Council and enlisted the help of the crew of the Audacity to transform a small shield world in its construction phase into Bastion and make it what it needed to be to nurture this eventual new species. The Librarian had the starship Eden built to carry the Precursor seeds and blooms to a world outside of the Milky Way galaxy that is ideal for planting and growth and where they can't be reached by the Flood. There, in some distant future, life on the planet will emerge and grow sentient, following the complete genetic code of the Precursors and rebirthing the race. However, they will be utterly free of genetic memory and will be able to build a new civilization with a clean slate. Bastion became the Librarian's secret laboratory away from the Ecumene Council and their rules and regulations containing the Librarian's most dangerous research and experiments. This included studies and experimentation on ancient humanity, the Forerunners and the Precursors, ingenious observation and trials, studies with Living Time, working Composers, Cryptums and other endless mysteries.
In the final days of the Forerunner-Flood war, Keeper-of-Tools, Birth-to-Light, Dawn-over-Fields and Clearance-of-Old-Forests are sent to be the caretakers of Bastion, but the Flood's advancement through Forerunner space changed things. The four Forerunners are waylaid time after time, cast into one battle after another and constantly evading capture. Knowing that their work on Bastion was too dangerous to fall into Flood hands, the four upload their essences into the Domain until the war is over and the way to Bastion can be achieved in secrecy and safety.
On September 1, 2557, Spark meets with a personality imprint of the Librarian a Forerunner complex beneath Mount Kilimanjaro Spark suggests that after humanity is given the tools to achieve the Mantle of Responsibility, there will be "Bastion," he could tell that the Librarian did not believe that she would ever make it there or to anywhere else where she would find rest and peace at last. With Spark choosing to remain with his friends with the Ace of Spades, the Librarian offered him a small etched box that she called a key with which he was to "find what's missing. Fix the path. Right what my kind has turned wrong."
Following Cortana accessing the Domain on Genesis, Keeper emerged from the Domain on the planet and left audio logs scattered across Genesis as he attempted to find his way to Bastion. Keeper determined that Bastion had been moved from its original location at some point even after slipspace had been destabilized by the deployment of the Halo Array, though Keeper believed such an act would require a nearly impossible act of causal reconciliation. Bastion was important enough that Keeper was willing to lose corporeal existence and resort to existing within a durance and connecting to the Domain to reach it, but was unable to find it due to it being moved, and instead became trapped on Genesis. In late 2558, the Builder accessed the Domain while Cortana and the Warden Eternal were distracted, and managed to finally confirm Bastion still existed. Keeper subsequently surmised that Bastion's safety measure had been activated at some point in the past, leading to it being moved.
At the same time, Spark and Rion Forge managed to locate Bastion in the Orion complex after passing a test given by the Precept. Their arrival momentarily opened Bastion's shields and transmitted a signal that Keeper was able to lock onto and use to transport himself into a monitor body on Bastion which was taken care of by monitors made out of indirect imprints of Birth, Dawn and Clearance whose real imprints may have been lost when the Domain was nearly destroyed by the firing of the Halo Array. With the danger of the Guardians chasing him to Bastion, Keeper decides to send the shield world into a slipspace portal to parts unknown, but he first enlists Rion and Spark's help to launch Eden to complete the Librarian's mission of ensuring the rebirth of the Precursors. Spark decides to remain behind on Bastion to act as its caretaker while Rion departs in the Ace of Spades. Bastion makes it through the portal without being followed and Spark initiates another jump to ensure that the shield world is far from any Guardian's reach while he begins going through the data left behind by the Librarian.
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