Riders of the Dead by Dan Abnett

In the brutal, war-torn world of the Old Empire, two soldiers of the Reiksguard, Gerlach and Dietz, find themselves stranded beyond enemy lines after a devastating battle against the Chaos hordes. Struggling to survive in a frozen wasteland haunted by madness and corruption, they are forced to question their loyalty, their faith, and even their humanity. As the borders between friend and foe, sanity and damnation blur, the soldiers must face an enemy more terrifying than any they have known—the corruption within their own souls. Riders of the Dead is a dark, gripping Warhammer tale of survival, brotherhood, and the unrelenting pull of Chaos.