Keyship

The Keyship 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. They were capable of remote detonation to prevent their usage by the Flood. Though many more once existed, the majority, if not all, were destroyed by the Librarian to prevent the Flood from reaching the Ark.

History
During the Forerunner-Flood War, there was an entire fleet of Keyships. It was used extensively by the Forerunners in their war against the Flood. A number of Keyships were given to the Librarian, and combined with her ability to build Portals, the Librarian was able to transport herself back to the Ark with her Keyships if the situation required. The Librarian, however, bravely destroyed her Keyships to prevent them from falling into the hands of the Flood, who with them, would be able to instantaneously travel to the Ark, rendering the Halo Array useless. Near the end of the war, the Keyships were part of an unknown strategy to hold off the Flood for nearly 79 years, although this advantage would be foiled by the massive computing power of the Flood. At the Battle of the Maginot Sphere, Didact was in command of multiple Keyships, although near the end, when the Forerunners began generating Slipspace ruptures to destabilize the battlefield and destroy Flood craft, Keyships were recklessly piloted near the rifts, destroying them.

The Forerunner Dreadnought 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 04B.