For too long had the Vth Legion ranged out beyond the sight of the wider Imperium, remaining ignorant of the Warmaster’s rebellion and the war that inevitably followed. Only once their primarch, Jaghatai Khan, had satisfied himself that the path before them was just and true did the White Scars choose a side, taking the fight to the traitors on every front. But, four years later, the Legion’s unfettered spirit has been broken by relentless attritional warfare against the Death Guard and the Emperor’s Children the Khan’s Stormseers must find a clear route to Terra if they are to take part in the final, apocalyptic battle.

Read it because
It's a brand new chapter in the Horus Heresy and begins the march to Terra though perhaps not in quite the way you'd expect.

The eBook edition includes integrated illustrations of characters and events from the story, and an afterword that places the story in context within the series.