The Persian by Barry Sadler (Casca Series, Book 6) from Books In Motion.com

The Brotherhood of the Lamb concluded its sacred meeting and the Brothers dispersed to their separate nations and cities with but one identical message: ""Casca lives, and is in Persia..."" The Eternal Mercenary enters the gates of the Persian Empire to become commander of Shapur II's royal forces. One man will seek his downfall at all costs, Rasheed, advisor to the King and secret member of The Brotherhood. When he watches Casca's burning flesh turn to ash, he knows not even the seemingly immortal Roman can survive the all-consuming hunger of the sacred fire...or can he?