Sigefrid Thurgilson was a Danish earl. He is the brother of Erik Thurgilson. We are first introduced to Sigefrid when Uhtred summons them to council for peace with Guthred.
Uhtred cuts off Sigefrid's sword hand after being sent to dispatch the brothers with Young Ragnar. Sigefrid repaces his sword hand with a metal dagger gauntlet.
Eventually, this breeds a deep hatred for Uhtred, and after returning from Frankia (France) with men to serve them, the brothers develop a plan to capture and kill Uhtred.
The brothers align with Guthred in Cumbraland for a time, but when Aelfric, uncle of Uhtred discovers that he was sold into slavery when Guthred had promised him Uhtred's head and leaves, Sigefrid and Erik leave as well.
Sigefrid and Erik then capture Lundene and and await the Anglo-Saxon assault. When Uhtred and Aethelred are dispatched to retake Lundene, Sigefrid and Erik pull out all their men and make for the Anglo-Saxon camp.
The brothers capture Aethelflaed (daughter of Alfred) and plan to ransom her off for gold and silver enough to create an army to rival that of Alfred. However, Erik falls in love with Aethelflaed and attempts to escape with her.
Sigefrid kills Erik to protect their wealth but fails to stop Uhtred, Finan and others from escaping the fortress with Aethelflaed. Sigefrid charges into battle with the Devonshire fyrd and the army of Wessex after Alfred meets Odda outside the fortress to stop his attack.
Sigefrid makes his final stand by duelling with Uhtred, but in the end Aethelflaed kills him with a sword through his heart.