Titandeath by Guy Haley

Horus's armada gathers, and he has defeated all enemies sent against him, even the Emperor's own executioner. One barrier remains before he can strike for Terra and lay waste to the Emperor's dream.The Beta-Garmon system occupies the most direct and only viable route to the Solar System and Terra. To break it, Horus assembles a war host of incredible proportions and Titans in untold numbers. To lose here is to lose the war and Horus has no intention of turning back. But the Imperium understands the importance of Beta-Garmon too. A massive army is arrayed, comprised of near numberless Army cohorts and a mustering of Titans to challenge even the martial might of the Warmaster.Titans fight against Titans as the God-Machines of Loyalists and Traitors alike go to war. This conflict will be like no other before it, a world-ending battle that will determine the next phase of the war.