Battle for the Abyss: My Brother, My Enemy

Horus sends the Worldbearer space marines to the planet Calth, where they are to ambush the loyalist Ultramarines. In addition to the main fleet, Horus sends a new doomsday battleship for use againsy the Ultramarines home world of Ultramar. A small strike force travels space and the warp to delay or destroy the doomsday ship to save the Ultramarines.