Blood Reaver

Driven on by their hatred of the False Emperor, the Night Lords stalk the shadows of the galaxy, eternally seeking revenge for the death of their primarch. Their dark quest leads them to a fractious alliance with the Red Corsairs, united only by a common enemy. Together with this piratical band of renegades, they bring their ways of destruction to the fortress-monastery of the Marines Errant.

Read it because
Aaron Dembski-Bowden takes an event mentioned in a Warhammer 40,000 rulebook the Fall of Vilamus and weaves his Night Lords characters into it, crafting a tense and dramatic tale of honour (or the complete lack thereof) amongst traitors... you'd think they'd know better that to trust one another.