The Janus Key, in the possession of the Librarian.
The Janus Key is a two-piece Forerunner Artifact. When combined, the key gives the location of every piece of Forerunner technology in the galaxy.[1]
Created in order to catalog the Forerunners' vast technological advancements, the key was placed in Requiem under the care of the Librarian's stored memory there. The key was to be presented to the Didact after he re-awoke in order to give to humanity. However, the Librarian eventually gave the key to Catherine Halsey in 2558 during the Requiem campaign. After she reawakened, Jul 'Mdama was able to take one half of the artifact and escape with Halsey, but not before she threw the other half of the key to SPARTAN-IV Gabriel Thorne.[2]