The Shadow Wolves are (or rather were) a chapter of the Adeptus Astartes.
They were listed as being successors of the Imperial Fists but author Aaron Dembski-Bowden, the creator of the chapter, has confirmed that they were meant to represent the concept of loyalist Luna Wolves as a tribute to his wife Katie (who is a fan of the Luna Wolves).
One of their strike cruisers, the Purest Intent, is captured by the Orks.
The last of the Shadow Wolves are slain by the Tyranid xenos at the battle for Varadon, the chapter's homeworld, a few weeks after the chapters monastery had been overwhelmed. The chapters survivors attempted to link up with the Black Templars but were surrounded. The last battle brother alive planted the chapter's banner even as he died.
All information above comes from the recollections of Reclusiarch Grimaldus
Helsreach: Warhammer 40,000