Now that the news of Horus's Treachery is in the open, a time of testing has come. Some Legions have already declared allegiance to the Warmaster, while the loyalty of the others lies firmly with the Emperor. As Horus deploys his forces, loyalist Astartes learn that the Wordbearers are sending a fleet 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. Battle for the Abyss continues the epic tale of the Horus Heresy, a galactic civil war that threatened to bring about the extinction of humanity.