By the power of truth, I, while living, have conquered the universe.
Posted by: burritosenior
Posted by: ajw34307
We are never told what this "gamechanger/latchkey" artefact was beyond a convenient plot device to shoehorn in nostalgic imagery surrounding the Forerunners, Cortana and Keyes/Pillar of Autumn designed to make the battle of Reach more important than it actually was.It wasn't explained perfectly well in the game, but Halsey's Journal clarifies a lot of things.
Nothing about the Latchkey Artefact beyond a vague mention or two. The journal ends before Noble Team reaches her under SWORD Base - at which point the data is still being decrypted, so we still don't know what it is or what it does.