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Pale Guardian
Book 7 in the James Asher Vampire Novels series.
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During the First World War, Lydia Asher serves as a nurse near the front and discovers that someone has learned to control revenants, the mindless undead feared even by vampires. Military authorities see the creatures as soldiers who obey orders, survive wounds and require no ordinary supply line.
In London, James investigates revenants appearing in Britain while European vampires feed unnoticed among the battlefield dead. Humans and vampires form an unstable coalition to locate the source of the creatures, knowing that a weapon designed for the trenches could multiply beyond any government’s ability to command it.