Blood and Thunder - Dan Abnett

Once in a generation, a new ork leader emerges, one mighty enough to unite the clans and unleash armies millions strong upon the Imperium of Man.
The green-skinned orks are the most warlike race in the blood-soaked galaxy of the 41st millennium. Cruel, crude and brutally barbaric, their unflinching passion for violence is so unquenchable that ork clans spend as much time warring with each other as they do with their enemies. And yet, once in a generation, a new ork leader will emerge, mighty enough to lay waste to his rivals and unite the clans, unleashing armies millions strong upon mankind. Destiny beckons, and it is from the ranks of the Blood Axe clan that the next call will be answered...