Halo: Starside Intercept is a Homeworld 2 total conversion bringing tactical space combat to the Halo universe.
The Keyship is a dreadnought class warship constructed by the Forerunners as a last ditch defensive option against the encroaching Flood. More important than its advanced weaponry is its ability to actuate Tier 1 megastructures, such as the Ark Portal. Without it, the Flood would be unable to locate and destroy the Ark, and all life preserved on it.
It will primarily be used in the Capture the Flagship gametype as an attainable derelict vessel, in addition to a map-specific object on the High Charity level. Little is known about what it does, or how it works, however it has a really big gun on the front that can blow up damn near anything.