David Annandale
Curse of the Wulfen

The Space Wolves' greatest secret is revealed! The mysterious 13th Company emerge from the warp, and the future of the Chapter is imperilled. Can the sons of Fenris survive the revelation of the Wulfen?
Warhammer 40k
novel

Also in:
Legacy of the Wulfen
Tales From Warzone Fenris
Sagas of the Space Wolves