Blood for the Gods by Maciej Szymczak

An elite unit of Slavic berserkers called the Wolves of Chernobog is devastating Danish lands. Slavic pirates are preparing to attack a Danish city. The defense is commanded by Jarl Sven, a recent Christian convert... The old gods rise to take part in a bloody battle. Let yourself be carried away by an amazing story in which pagans face Christians in a fight to the death.