Key Ship
From Halopedia, the Halo Wiki
The Keyship[1] is a classification of supra-luminal Forerunner naval vessel in use at the time of the Forerunner-Flood War.
Mentioned only in the hidden Terminals found on the Ark and second Installation 04, Keyships were a class of Forerunner vessel capable of activating portals leading to the Ark. Though many more once existed, the majority, if not all, were destroyed by the Librarian to prevent the Flood from reaching the Ark.
The Forerunner Dreadnought of Halo 2 and Halo 3 is most probably one of, if not the last surviving Forerunner Keyships in existence, used by the Covenant for an unknown amount of time as an unusual source of power for their Holy City, High Charity. At the end of the Ninth Age of Reclamation the Prophet of Truth appropriated the Dreadnought to activate the Portal on Earth and travel to the Ark. It was last known to have gone to ground on said Installation, and is believed to have been destroyed upon the premature activation of Installation 04 (II).
They were capable of remote detonation to prevent their usage by the Flood.
