Key of Osanalan
From Halopedia, the Halo wiki
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. From Ackerson's tortured admission it is implied that the previous Halos failure may have been due to the absence of this Key.
