Q. What happens when a viking gets bit by a radioactive spider?
A. The radioactive spider gets viking powers.
Brian Wood's Northlanders contains very little of the fantastic elements common in most comic series. There's no superpowers or zombies. And while Norse mythology comes into play in some story arcs, it serves to add flavoring and background. For the most part, this is real, raw history. But when it's history of a culture as brutal and beautiful and fascinating as the Norse, all that other stuff just gets in the way.
Also unique for an on-going series, Northlanders doesn't focus on a particular cast of characters. Instead, Wood said in an interview, "From day one it was always my desire to cover as much ground as possible. Right now, I have stories that span from 760-1100 AD, from Russia to Greenland, from the perspective of men, women, and children, from the Vikings themselves to the people they conquered (or tried to). That breadth of story is what makes the book what it is."
Since Northlanders is an anthology series, you can jump in at just about any point. However, I do recommend the graphic novels The Plague Widow and Metal and Other Stories as excellent places to start.