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 Tyranids at the battle for the chapter's homeworld of Varadon, a few weeks after the Chapter Monastery had been overwhelmed. During the battle, the chapter's surviving members attempted to link up with the Black Templars, but were surrounded and cut down by the hordes of Tyranid organisms. The last living battle-brother planted the chapter's banner even as he was torn apart by the Tyranids.
999 M41 A strike team of Shadow Wolves arrived on Cadia to help its defence just prior to the Fall of Cadia during Abbadon's 13th Black Crusade.