Book 4 in the Danelaw Saga by Griff Hosker.

King Æthelred was the worst King of England but, in his second son, Edmund Ironside, was a king in the mould of King Alfred. Prince Cnut has his own problems for his brother refuses to share the crown of Denmark with him. It is Sven Saxon Sword, wielding Oathsword who orchestrates the campaign that brings the Danish fleet to invade England. The oath he made on the sword is now a fetter, tethering him to the fate of Prince Cnut of Denmark.