- “I've remotely destroyed our Keyships. A security measure. Without them I cannot reach the Ark. But neither then can the thing.”
Mentioned in the hidden Terminals found on the Ark and Installation 04B, 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 were destroyed by the Librarian to prevent the Flood from reaching the Ark.
During the Forerunner-Flood War, there was an entire fleet of Keyships, used extensively by the Forerunners in their war against the Flood. A number of Keyships were given to the The Librarian, and combined with her ability to build Portals to the Ark, the Librarian was able to transport herself back to the Ark with her Keyships if the situation required. The Librarian, however, 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, Ur-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.
Key to RepopulationEdit
Keyships were also an integral part of the Forerunners' "Conservation Measure" to preserve sentient life. Before the firing of the Array, the Forerunners gathered DNA and living specimens of the notable sentient species located throughout the galaxy and protected them from the devastation of the Flood and all other life via Halo by transporting them to the Ark. After the seven ringworlds fired out their pulses, several Keyships, containing preserved specimens of each species, were sent to the planets from which the sentient species originated, and released a population of the species on their planet of origin. These ships therefore were not only a vessel of war, but also designed to reinstate life on the planets devastated by Halo.
The rampant Forerunner AI 032 Mendicant Bias would find his way on board a Keyship delivering the San 'Shyuum back to their homeworld. By 2552, that particular vessel, commonly referred to as the Forerunner Dreadnought, was most probably one of, if not the last surviving Forerunner Keyship 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 Installation 00. 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.