Phalanx

The Phalanx is the Imperial Fists' fortress-monastery, a moon-sized warship that predates the Imperium and is often stationed over Terra. Rogal Dorn offered it to the Emperor, who told him to keep it as the VII Legion's seat. It fought through the Solar War, the fall of Cadia, and later Tyranid actions, and it remains a mobile station rather than a true Sol native.