In 922 A.D. an emissary from Persia, Ahmed Ibn Fahdlan (Antonio Banderas) is banished from his land for having an affair with the ruler's wife and finds himself in Northern Europe. He accompanies a group of 12 Vikings who are on a mission to help another kingdom besieged by what they describe as demons. It's all vaguely like the BEOWULF legend, though I find this more interesting. The first part of the movie leaves it up in the air if they are supernatural creatures or not and then becomes even more interesting when we find out the truth. It turns out that the attackers are a vicious tribe of primitive humans, like Neanderthals, that worship bears and dress in their skins.
This is a really good adventure film with rousing battles and a sense of humor.
Based on the book, THE EATERS OF THE DEAD, by Michael Crichton. It's one of those rare cases where the movie is far far better than the original source material.