Anodyne Spirit

The fan named Forerunner Ship was used by the Covenant to power all of High Charity. When the Flood attacked the city, the Prophet of Truth uncouples the ship from High Charity's power grid, and uses it to travel to Earth. The Forerunner ship travels vertically, like a rocket. It is also capable of slipstream travel. It is also possible that this ship could be the "Ark". However this is unlikely for the Prophet would have surely used the Ark to activate the remaining Installations as soon as they went on standby after Installation 05 was deactivated.

A ship with enough power to power all of High Charity could likely move faster than any other ship in existence, which is quite impressive for a 100,000+ year old machine.

At the end of Halo 2 Master Chief gets onboard this Forerunner Ship, in his mission to kill the High Prophet of Truth. It is possible there is a Foreunner A.I. in the ship. This ship is very geometric and angular in its design, and is approximated to be about 14 kilometers tall.