Eaters of the Dead by Michael Crichton
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The year is A.D. 922. A refined Arab courtier, representative of the powerful Caliph of Baghdad, encounters a party of Viking warriors who are journeying to the barbaric North. He is appalled by their Viking customs-the wanton sexuality of their pale, angular women, their disregard for cleanliness...their cold-blooded human sacrifices. But it is not until they reach the depths of the Northland that the courtier learns the horrifying and inescapable truth: he has been enlisted by these savage, inscrutable warriors to help combat a terror that plagues them-a monstrosity that emerges under cover of night to slaughter the Vikings and devour their flesh....