Siegemaster

On the world of Harrowar, siege lines and armies span the entire planet... but after years of preparation, war is yet to begin. Warsmith Bakulos arrives to take command of the Iron Warriors and ensure that the continent-spanning siege becomes a war that he can win. But his old comrade Kurr doesn't see it that way, and Bakulos remembers the events that have led them both to this place and the final confrontation that inevitably awaits them.

Read it because
It shows the fractured and broken Iron Warriors through the eyes of one of their own. Bakulos' reminisces of what once was and how things came to be are compelling, the hints of the tragic events of the Horus Heresy and afterwards intriguing... and the ending is as close to heartbreaking as Warhammer 40,000 gets.