Sword of the Emperor

Gaius Galen is a legionary assigned to a legion at Germania, the Northern Frontier. Rome is attempting to conquer the Germanic tribesmen and make them part of the Roman Empire. During the final battle, when the two great armies clash, Galen saves the life of General Petronius Valens, the commander of all the Roman legions.
After the great battle, General Valens and Galen are called back to Rome. In Rome, Galen saves the emperor, Marcus Aurelius from an assassination attempt. The emperor makes Galen one of the Riders, exceptional men who answer only to the emperor and ride to hot spots throughout the empire, fighting bandits, murderers, and thieves who are victimizing the local population. They do the job that a clumsy large legion cannot do. Riders have the power of life or death. If they cannot bring the evildoers to justice, then they are to kill them, thus ending the problem.