Keyship

During the Forerunner Flood war, the Forerunners created numerous Portals to the Ark(Installation 00) all of which required the use of Key Ships to be activated, as a preventative measure of the Flood activating it.

As a secondary preventative measure, all Key Ships were capable of being remotely destroyed, to prevent them from being captured by the Flood. As one of the final measures before the Rings where fired, the Librarian remotely destroyed the Key Ships, sealing him/her self from the Ark, and his/her lover.

Truth`s Forerunner Ship is in fact a Key Ship which appears to have not been destroyed. It was required to activate the Portal on Earth, a process in which it connected itself to the Portal, causing it to become online, then when the Portal began to open, it channeled energy into the sky, to create a Portal to the Ark.