Long before the grim darkness of the 41st millennium was a time of myth and legend, when god-like beings met in a monumental battle of brother against brother. Ten thousand years prior to the Battle of Macragge, where the 1st Company of the Ultramarines bravely gave their lives defending against the Tyranid hordes of Hivefleet Behemoth, and ages before Abaddon the Despoiler conceived of his heinous and bloody Black Crusade, occurred the greatest of all battles. Let us return to the Imperium of the 31st Millennium, when a golden age was shattered by betrayal. Welcome to the Horus Heresy.