The Broken Crown (Legacy of Russ #4)

The Fenris system is under attack by armies of daemons, and though the Space Wolves fight with their customary courage and savagery, their enemies are beyond number. In an effort to quell the daemonic intrusion, Azrael orders the firebombing of the planet Midgardia, in callous disregard for the fate of its human inhabitants. And in the void, the Inquisition discover that while rumours of mutants among the Space Wolves are true, the reality of the situation is not clear cut as it could be – are the Wulfen friend or foe?

Read it because
It's a turning point for the Space Wolves as the Dark Angels and Inquisition reveal their own allegiances.