Dr. Halsey said Cortana had some sort of artifact or game changer that would help defeat the Covenant. Did I miss something, or was this never explained? What was she on possession of?
Coordinates of Installation 04 (that “random jump” wasn’t so random), and likely other things such as extensive compatibility with Forerunner computer systems. (Including the ability to store and conceal a copy of 04’s Index in her own memory.)