Mechanicum by Graham McNeill

As the flames of treachery spread throughout the Imperium, Horus plots to subvert or destroy all those who would stand against him. On Mars, home world of the Mechanicum priesthood, the great manufactory-cities have long produced much of the weaponry required for the expeditionary fleets across the galaxy – making the world invaluable to whoever controls it in the coming war. Now, the Warmaster's agents begin to stoke the fires of rebellion, turning the loyalist forges and the mighty Titan Legions against one another. And, with whispers spreading of an ancient terror lurking beneath the Red Planet's surface, the Dark Mechanicum rises...