Betrayer by Aaron Dembski-Bowden

A Horus Heresy novel As Calth burns, the Word Bearers and World Eaters begin a bloody crusade across Ultramar. Worlds will burn, legions will clash... and a primarch will fall. READ IT BECAUSE It's finally revealed why Lorgar didn't lead the assault on Calth... because he's burning his way across Ultramar. But why, and why is Angron with him? Mysteries abound, and primarchs meet on the battlefield... THE STORY The Shadow Crusade has begun. While the Ultramarines reel from Kor Phaeron's surprise attack on Calth, Lorgar and the rest of the Word Bearers strike deep into the realm of Ultramar. Their unlikely allies, Angron and the World Eaters, continue to ravage each new system they come across - upon the garrison planet of Armatura, this relentless savagery may finally prove to be their undoing. Worlds will burn, Legions will clash and a primarch will fall.