Janus Key

"We did not create so much without a means to catalog it all."

- The Librarian's imprint to Catherine Halsey

The Janus Key is a Forerunner cartographical artifact that provides real-time location of every piece of Forerunner technology in the galaxy when its halves are combined.

Background
Created in order to catalog the Forerunners' vast technological advancements, the key was placed within the Librarian's Rest inside the shield world Requiem; there, it would remain under the care of the Librarian's locally stored memory. The device was to be presented to the Ur-Didact after he returned from stasis so he could give it to humanity to spur their technological advancement. However, the Didact turned on humanity when he was awakened in July 2557, renewing his quasi-genocidal campaign which had been halted one hundred millennia prior.

History
Six months after the Didact's attack on Earth was foiled, the Librarian's stored personality gave the key to Dr. Catherine Halsey, instructing her to take the key to the Absolute Record. After Halsey reawakened in the Covenant-controlled Copernicus Base, Jul 'Mdama took half of the artifact and escaped with the doctor. As she was being carried away, she threw the other half of the key to Spartan Gabriel Thorne. Thorne eventually gave his half to Dr. Henry Glassman for further study.

In July 2558, five months after the Requiem Campaign, Glassman continued his inspection of the incomplete Janus Key onboard the. At the same time, Dr. Halsey, who allied herself with Jul 'Mdama, plotted to acquire the UNSC's half of the Janus Key. She and 'Mdama constructed a decoy Forerunner artifact for Infinity to find on the surface of the planet Aktis IV. Upon its discovery, a deceived Dr. Glassman requested Infinity to transport the UNSC's half of the key down to the planet, to examine how the two artifacts would interact. Captain Lasky dispatched a Pelican and a Broadsword to deliver the key to the site. As the aircraft approached their destination, they were both shot down by Covenant anti-air artillery. The attack, however, was not issued on Jul 'Mdama's order and was instead carried out by defectors of 'Mdama's own faction. Infinity ' s half of the Janus Key immediately fell into the hands of Jul 'Mdama's leading detractor, a self-proclaimed Sangheili prophet named Sali 'Nyon.

However, a Sangheili Minor disillusioned with 'Nyon's rebellion stole his half of the Janus Key and contacted 'Mdama's forces, offering them the key along with the location of 'Nyon's hideout. A contingent of 'Mdama's troops met with the Minor, with Dr. Halsey personally verifying the artifact's authenticity. However, after 'Mdama redirected most of his forces elsewhere, three Spartan-IVs led by Commander Sarah Palmer ambushed the group. Halsey attempted to escape with the Janus Key half but she was taken captive by Palmer. However, the doctor was able to distract the Spartan commander, escaped with the artifact, and was picked up by one of 'Mdama's Phantoms. Now in possession of both halves of the Janus Key, 'Mdama and Halsey recombined the artifact, generating a holographic map pinpointing the location of the Absolute Record.

Trivia

 * The key is named after the Roman god Janus, who had dominion over beginnings and endings. The key's being split in half is also a reference to Janus, as he is usually depicted with two faces.
 * The Janus Key is similar in purpose to the quasi-extant Organon, which some Forerunners believed would activate all Precursor technology in the galaxy.
 * During the filming of motion capture for Episode 9's cutscene The Key, two highlighter pens were used as props for the two halves of the Key.
 * Each half of the Janus Key is shaped after the center part of the Forerunner glyph for the Mantle.
 * The contested Janus Key half formerly aboard Infinity inexplicably changes color between issues 14 and 15 of Halo: Escalation. While originally depicted as blue, the half is red in issues 15 and 16.

List of appearances

 * Halo 4
 * Spartan Ops
 * Halo: Escalation
 * Halo 5: Guardians