Unaware of the wider Heresy and following the Warmaster’s increasingly cryptic orders, Roboute Guilliman returns to Ultramar to muster his Legion for war against the orks massing in the Veridian system. Without warning, their supposed allies in the Word Bearers Legion launch a devastating invasion of Calth, scattering the Ultramarines fleet and slaughtering all who stand in their way. This confirms the worst scenario Guilliman can imagine Lorgar means to settle their bitter rivalry once and for all. As the traitors summon foul daemonic hosts and all the forces of Chaos, the Ultramarines are drawn into a grim and deadly struggle in which neither side can prevail.

Read it because

It's the beginning of one of the Horus Heresy's most infamous and epic battles. Dan Abnett's story grabs you by the shoulders and throws you into the midst of the Betrayal at Calth. The moment when the build-up ends and the battle begins is breathtaking and brutal and the rest of the novel doesn't let up.

The eBook edition includes integrated illustrations of characters and events from the story, and an afterword that places the story in context within the series.