The Black Chalice — Marie Jakober

The year is 1134. In an isolated German monastery, an aging monk begins the history of a war. It is in part his own story, for he was there, thirty-one years before, as squire to the proud Crusader knight Karelian of Lys.
But even as Paul begins his history, a strange encounter with the Otherworld enchants his quill. He can no longer write the cautionary Christian tale his superiors expect; he must write what he truly remembers.