Key Ship

First Mentioned in the various Terminals scattered in the Halo 3 levels The Ark, The Covenant, and Halo, these ships are able to activate various portals that can allow the Forerunners to reach the Ark. There are at least two key ships implied to have existed but there is a strong thought that there are more due to one of the ships having been destroyed. The last ship was in possession of a character referenced as L in the terminals. This L character was responsible for firing the Halo arrays and refers to the Librarian.

The description of a ship capable of activating a portal to the Ark fits the Covenant's converted Forerunner Dreadnaught exactly. Anything with that much energy would easily power High Charity.