War of the Fang by Chris Wraight

A thousand years after the Heresy, the Space Wolves find themselves outgunned as the Thousand Sons invade Fenris.

Contains the prequel novella The Hunt for Magnus and the novel Battle of the Fang.

The Hunt for Magnus
It is the end of the thirty-second millennium. For two thousand years, since the fall of Prospero, the Space Wolves have hunted their greatest foe, the quarry who escaped them on that benighted world: Magnus the Red, sorcerer-primarch of the Thousand Sons. Now, Great Wolf Harek Ironhelm has the traitor primarch’s scent... Determined to finish what Leman Russ began, Ironhelm sets out to capture and kill his prey, and the Hunt for Magnus begins.

Battle of the Fang
It is M32, a thousand years after the Horus Heresy. The Scouring is over and the Imperium at the height of its post-Crusade power. When Magnus the Red is tracked down to Gangava Prime, the Space Wolves hasten to engage the daemon primarch. Even as Great Wolf Harek Ironhelm closes on his ancient enemy, the Fang on the Space Wolves home world is besieged by a massive force of Thousand Sons. A desperate battle ensues as the skeleton forces of Wolf Lord Vaer Greylock attempt to hold back the attacking hosts before the last of his meagre defences gives in. Though a single Scout ship survives to summon Great Wolf Harek Ironhelm back to Fenris, none of the defenders truly realise the full scale the horror that awaits them, nor what the Battle for the Fang will cost them all.