Now that news of Horus' treachery is in the open, a time of testing has come. Some Legions have already declared their allegiance to the Warmaster, while the loyalty of others lies firmly with the Emperor. As Horus deploys his forces, loyalist Astartes learn that a massive Wordbearers fleet is heading to Ultramar, home of the Ultramarines. Unless they can intercept and destroy it, the Ultramarines may suffer a blow from which they will never recover.