The Shadow Within by Serra Rose

Two brothers. Two realms. One inescapable fate.
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In 14th Century England, two brothers reunite after years of being separated by war. Thomas and Graeme are haunted, in more ways than one: Thomas is plagued by memories of the atrocities he committed on the battlefield, while Graeme suffers from disturbing nightmares he does not understand.
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When a sinister killer known as 'The Shadow of Death' begins to stalk the nearby town of Kempschester, Thomas is conscripted to the Enforcers. But while Thomas is setting out to hunt down this malevolent force, Graeme is discovering his own mysterious connection to this alluring murderer.
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As Graeme is drawn into a new realm of darkness and Thomas learns that the killer may be closer to home than he thinks, the brothers find themselves on a collision course with not just each other but an entirely new world - and this world has hungers of its own . . .
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Prepare to witness the origin of The Grim Reaper