Book 7 of the Ciaphas Cain series

Legendary Commissar Ciaphas Cain joins a group of Reclamators Space Marines aboard a space hulk, but soon finds himself alone and faced with both tyranids and orks…

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It's a typically sideways look at the mighty Adeptus Astartes (albeit a pretty unusual Chapter), viewing them through the eyes of the wry and dry Commissar Cain.