The Mongol by Barry Sadler (Casca Series, Book 22) from Books In Motion.com

Cursed by Christ on Golgotha, condemned to outlive the ages, and wander the globe a constant soldier. Forever fighting, surviving, waiting for Him to return. A slave of the savage Tartars, Casca is a champion in the blood sports that make his masters rich...a rare prize for those who wager on his skills...and a nightmare for those who face him in a fight where only one survives. Then Casca is stolen by a Mongol rebel, a young outcast among his own people. Only blood and power can quench the burning thirst of his ambition. With Casca at his side, he unites a people and cuts a bloody swath across Asia into Europe. Nations fall beneath an army horde that knows no mercy. And an empire rises from the slaughter.