Key of Osanalan
From Halopedia, the Halo wiki
The Key of Osanalan is a fictitious Forerunner object underneath the city of Cleveland, Ohio on Earth.[1] Colonel James Ackerson said that without the Key, the Halos will not properly activate.
However, it the key does not exist at all. During their childhood, James Ackerson and his brother, Ruwan, made up an object called the Key of Osanalan for their make-believe games.[2] It is likely Ackerson told the Covenant about the key to prevent them from simply destroying Cleveland.
