Eaters of the Dead — Michael Crichton

In the year A.D. 922, a refined Arab courtier, representative of the powerful Caliph of Bagdad, encounters a party of Viking warriors on their journey to the barbaric North. He is appalled by Viking customs--the wanton sexuality of their pale, angular women, their disregard for cleanliness, their cold-blooded human sacrifices. But only in the depths of the Northland does he learn the horrifying truth: He has been enlisted to combat a terror that comes under cover of night to slaughter the Vikings and devour their flesh....