The Barbarian by Barry Sadler (Casca Series, Book 5) from Books In Motion.com

The Plains of Parthia were littered with the bodies of the dead and dying, as Casca, the man who could not die, moved away from the battle. Rome did not take desertion lightly, and while they couldn't kill The Eternal Mercenary, they could make his life a nightmare of pain and torture. His only escape lay in the Northlands beyond Germany, to lands filled with beautiful fjords and horrible bloodlust. Casca the Roman Citizen must become Casca the Barbarian.