I obtained a copy of Black Monastery from the author.
Black Monastery is a historical horror novel by former soldier William Stacey. Set at the end of the 6th century, it tells the story of a Viking raid gone horribly awry. Asgrim Wood-Nose, Viking war-chief and outlaw, sails against the Franks, raiding to raise the fortune he needs to amass in order to pay his blood-debt and return home. On the island of Noirmoutier, rather than the promised silver, he finds evidence of a horrible massacre… and an ancient lurking evil.
You can read the full review over at Book Nouveau. It contains mild spoilers.