Janus Key

The Janus Key is a Forerunner 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 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 2557, renewing his quasi-genocidal campaign which had been halted one hundred millennia prior.

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.

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 fictitious 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.

List of appearances

 * Halo 4
 * Spartan Ops