Key of Osanalan
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The Key of Osanalan is a Forerunner object underneath the city of Cleveland, Ohio on Earth. [1] 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 single detonations, as a sort of stop-gap measure to insure 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.
