Janus Key

"This is the Janus Key. it offers the real-time location of every piece of Forerunner technology in the galaxy."

- The Librarian to Dr. Catherine Halsey

The Janus Key is a device containing a complete catalogue of every piece of Forerunner technology in the entire Milky Way Galaxy.

The Janus Key was created to track and offer real-time locations for every piece of Forerunner technology within the Forerunner's technological empire (as far as Halo canon extends, it stands to be only within the Milky Way Galaxy).

Librarian's Gift
Jul 'Mdama somehow knew that the Librarian's Shrine contained within it a  valuable artifact. However he believed that the gift would be knowledge directly from the Librarian and had no idea it was a material gift in the way of a 'Key'. When 'Mdama successfully kidnapped Dr. Halsey, with the aid of Promethean Knights, he knew that she could activate the Shrine for him, due to her own personal quest for immense knowledge. When Catherine deactivated the shield surrounding the Shrine, it activated, and she quickly entered it, to the anger of 'Mdama who wanted to be the one and only being to converse with the Librarian.

Catherine soon was face to face with the Librarian, the one responsible for reseeding the galaxy after the Halo Array was fired. The Librarian told Catherine to take the key to the Absolute RecordAbsolute Record and that she must accept her gift as Requiem's time was coming to an end and this knowledge would be lost, severely damaging humanity's chance of reclaiming the Mantle. When Catherine accepted this gift she was propelled back into reality and Jul 'Mdama's grasp. 'Mdama soon relieved Halsey of one of the two pieces of the Janus Key, the blue piece of the Key, unaware that there were indeed two pieces. During this time, Spartan Gabriel Thorne entered the Shrine hidden by Gek's cloaking device, to which he assassinated one of 'Mdama's several bodyguards surrounding him and Catherine. Halsey immediately threw the orange half of the Key to Thorne. Directly after this Sarah Palmer appeared on her mission to assassinate Catherine, entering just as the chaos of the situation exploded. Jul 'Mdama, aware of the threat of an entire squad of SPARTAN-IVs, soon called for reinforcements in the way of the Promethean knights. During this small brush with Fireteam Majestic, 'Mdama quickly moved away from the battle while holding Catherine hostage. When the Spartans and Sarah Palmer had successfully eliminated the Prometheans, a lone Promethean Knight teleported behind 'Mdama and Catherine and transported them somewhere within Requiem, but not before Sarah managed to fire one round directly into Catherine's shoulder, only wounding her, not killing her, like her mission briefing instructed her to.

Sarah, enraged with 'Mdama escaping with Halsey for a second time, flew into a tempered rage, only stopping to question Fireteam Majestic and berate them for attempting to rescue Doctor Halsey, for Thorne to then question why Halsey deemed the artifact in his possession to be of importance.

Usage
The key is comprised of two sections, one blue with orange detail and the other orange with blue detail. It is fairly representative of the ying-yang symbol as seen in human culture, and as the Librarian seems to favour blue and the Didact orange, it wouldn't be entirely unreasonable to speculate the blue side contains information on lifeworker artifacts and the orange, military equipment such as ships, halo rings, armouries, and Shield worlds such as Requiem and Onyx. If this is the case then the UNSC has the 'upper hand', so to speak, in terms of weaponry while the Covenant has information on artifacts and research. However this is not confirmed as yet.

Upon assembly, the device emits a 3D representation of the Milky Way Galaxy and within it the realtime location of every piece of Forerunner technology ever assembled. It is not known whether or not the device actually emits this map in our realm as when she was demonstrating its use to Halsey it is unclear whether or not she was doing so in Halsey's mind or an alternate dimension. It holds data and coordinates of all Forerunner technology. If the pieces are separated, the Key seems to be almost useless.

Trivia

 * The Librarian states that the Didact was supposed to use the Janus Key to aid humanity when he awoke from his meditation.
 * The name of the artifact is likely a reference to Janus, Roman god of gates and doors.