Key of Osanalan

The Key of Osanalan is a Forerunner object underneath the city of Cleveland, Ohio on Earth. As revealed by a captured Colonel James Ackerson, without the Key, the Halos will not properly activate. Outside of Halo: Uprising, no mention of the key is seen in Halo 3, and Master Chief activates the replacement Installation 04 successfully even without the key. One of the main characters in Uprising, Ruwan claims to know exactly what it is.

Speculation

 * The key, could be required, for a full Array activation, but not for singular detonations, as a sort of stop-gap measure and extension to ensure that the galaxy, is not wiped out by accident, or the key could likely be the activation index for the portal, which would explain its location on Earth and not a Halo ring. The key could also be a Keyship, much like Truth's Dreadnought.
 * The key, could be related to the Activation Index, whether in appearance, or not, it holds a striking semblance metaphor-wise.
 * The key could also be one of Mendicant Bias's Fragments, that is needed to activate the Portal to the Ark. It would explain why the Forerunner Dreadnought was just sitting in its Cradle, for such a long time.
 * The key, could be a master key of sorts, able to activate any of the halo rings, without their respective firing indexes.

Related Articles

 * Halo: Uprising
 * Battle of Cleveland