Dawn of Fire: The Wolftime written by Gav Thorpe.
The Indomitus Crusade has brought the Emperor’s vengeance to thousands of star systems. The fleets and armies under the leadership of Roboute Guilliman fight for the survival of humanity against the forces of the Chaos Gods. But the traitors and heretics are not the only foe looking to destroy the rule of Terra.
Xenos prey on human worlds in numbers not seen for millennia. Worst amongst them are the rampaging orks, whose migration conquests threaten to reverse the many gains of Fleet Primus. And their throaty bellows carry a name not heard in years, of destruction made flesh, a bestial warlord without peer: Ghazghkull Mag Uruk Thraka.
In the midst of this brutal tide is Fenris, the world of the Space Wolves. Depleted by ever-greater demands on their warriors, called upon by the Legion-breaker Guilliman, the Wolves of Fenris face a momentous decision. Grimnar and his counsellors must choose whether their fate is to ally themselves with an ancient rival and risk all that makes them the Vlka Fenryka or to accept their demise and wait for the return of their own primarch and the coming of the Wolftime.