Sabbat Worlds: Of Their Lives in the Ruins of Their Cities (Gaunt's Ghosts)

Dan Abnett takes the Tanith First back to the beginning. On Voltemand, long before they gain fame and glory as Gaunt's Ghosts, the Tanith are a broken unit. Crippled by doubt and hating their commander for his part in the death of their world, they are fractious and undisciplined. Leading a patrol into no-man's-land, Gaunt is ambushed. The Tanith must rally around and protect their new leader, or be consigned to history.

Read it because
It's a beautiful and elegiac tale, looking back at Ghosts long gone and providing a epitaph to many of your favourite characters from the long running series.