The Damned by Barry Sadler (Casca Series, Book 7) from Books In Motion.com

In a state of near despair, Casca leaves behind the now dreaded gates of the Persian Empire, and makes his way back to the Northlands. He seeks the walls of Helsfjord and the gates of The Hold. It was there, years before, that the Roman knew his happiest moments. Soon afterward he is found living among a pack of wolves as half-man, half-beast. For those unfortunate enough to stumble upon The Hold, he has become known as the Monster of the Mist, a primitive being who allows no man to escape his clutches alive. He is summoned back from his crazed state to face one of his most formidable foes, the would-be conqueror of all Europe, Attila the Hun.