Litany of the Destroyer by Chaz Lebel

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Sir Galien Banforth is damned. He watched a demon drag his lord to Hell, and he knows he’s next. When war once again pulls him into a theater of pillage and butchery, Galien abandons the battlefield, and his knighthood, to find a way to repent. Fate is about to give it to him. The dark cult of Apollyon the Destroyer grows by the day, its adherents spreading a catastrophic sickness. Galien is about to find himself in a new war, against the apocalypse itself, and the demons that carry it on noxious wings.