Battle for the Abyss by Ben Counter

Now that news of Horus's 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 the fleet, and destroy the mighty battleship that has been sent to reinforce it, the Ultramarines may suffer a blow from which they will never recover.