From Halopedia, the Halo wiki
A coordinate key is a piece of Forerunner technology that leads the user to a location and grants them access to it.
During 343 Guilty Spark's meeting with a personality imprint of the Librarian in a Forerunner complex beneath Mount Kilimanjaro, she provided him with a small etched box containing a coordinate key with the instruction to "find what's missing. Fix the path. Right what my kind has turned wrong."
Months later, when Lessa handled the coordinate key, it responded to the touch of a Reclaimer and part of it collapsed inwards by twelve millimeters to form the symbol for Installation 07. After six months without success in finding the UNSC Spirit of Fire, Rion Forge agreed to take the Ace of Spades to follow the trail set by the key.
On Zeta Halo, Spark inserted the coordinate key into the installation's Cartographer with Rion adding her touch as a Reclaimer. The Cartographer displayed a star map pointing to a location on New Carthage before ejecting the key and a second component which Rion combined together. A slipspace portal opened and pulled Rion through, although Spark managed to keep hold of the coordinate key. When Ram Chalva inserted the coordinate key into a terminal at the way station that the map had pointed to, it released a third component to the key. When the pieces of the coordinate key were put together, it pointed to Erebus VII with no spots for a fourth piece.
On Erebus VII, Spark placed the key into a terminal and it summoned a Monitor known as the Precept who revealed that he had initiated the slipspace portal after sensing that Rion was a Reclaimer and summoned her to Erebus VII to judge her through Rion's experiences on the planet with the Eiyaa-Mahtuhaa. After studying Spark, the Precept decided that he could proceed and tested Rion's biological memory. Satisfied, the Precept returned the coordinate key to Spark and Rion with coordinates and access permissions to Bastion for the two of them alone.
The coordinates led the two to a static slipspace portal in orbit of a moon in an unnamed star system which the coordinate key allowed them to pass through. The Ace of Spades emerged over Bastion, which proved to be a shield world in the Orion complex.
On Bastion, the Librarian's secret laboratory, Rion used the coordinate key to launch the Eden alongside a hard light key used by Keeper-of-Tools. Keeper told Spark that the key was meant to be brought to Bastion by the Librarian herself and that Rion's involvment was a last resort by the Librarian to enact her plans. Keeper revealed that the key, along with leading to Bastion, allowed unrestricted access to and control over Bastion and its caretakers. After Eden's departure, Spark used the coordinate key to assume control of Bastion as its new caretaker as Keeper had wanted.
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