Swords of the Legion (The Videssos Cycle, Book 4)

In Videssos the city, tribune Marcus Scaurus was bored. The legion that had been magically transported to this strange world was far away. But the Emperor's niece Alypia was near -- and willing.
When their secret trysts were betrayed, Emperor Thorisin Gavras was forced to condemn Marcus as a traitor -- but with a promise of freedom and Alypia, if he could reclaim a rebel province from a fanatic usurper, with no military aid. With only centurion Gaius Philippus, Marcus set out to try the seemingly impossible task.
But the fates conspired against them, driving them further westward, into the innermost sanctum of Videssos' great enemy Yezd -- and toward the torture chambers of the evil, deathless wizard-prince Avshar.
But behind them, without orders, the men of the legion were on the march!