Sword Song by Bernard Cornwell (The Last Kingdom #4)

The year is 885, and England is at peace, divided between the Danish kingdom to the north and the Saxon kingdom of Wessex in the south. Warrior by instinct and Viking by nature, Uhtred, the dispossessed son of a Northumbrian lord, has land, a wife and children—and a duty to King Alfred to hold the frontier on the Thames. But a dead man has risen, and new Vikings have invaded the decayed Roman city of London with dreams of conquering Wessex... with Uhtred’s help. Suddenly forced to weigh his oath to the king against the dangerous turning tide of shifting allegiances and deadly power struggles, Uhtred—Alfred’s sharpest sword—must now make the choice that will determine England’s future.