Long before the Imperium's arrival, the Realm of Ultramar was ruled by Roboute Guilliman, the last Battle King of Macragge. Even after learning of his true lineage as the son of the Emperor of Mankind, he sought to expand his dominion as efficiently and benevolently as possible, with the XIII Ultramarines Legion at his sole command. Now, facing a rival empire on the Ork world of Thoas, Guilliman must choose his weapons carefully—or the dream of a better future may be lost forever.