Empire of Dragons - Valerio Massimo Manfredi

The town of Edessa, a Roman outpost, is on its last legs, besieged by the Persian troops of Shapur I. Roman Emperor Licinius Valerianus agrees to meet his adversary to draw up a peace treaty, but it is only a trap and the Emperor and his twelve guards are chained and dragged away to work as prisoners in a solitary Persian turquoise mine.

After months of forced labour the Emperor dies, but his guards make a daring escape lead by the heroic and enigmatic chief, Marcus Metellus Aquila. They meet a mysterious, exiled Chinese Prince, Dan Qing, and agree to safeguard his journey home to reconquest his throne from his mortal enemy, a eunuch named Wei.

Thus begins the adventures of the Romans and the Prince as they journey to China. There they will discover that they aren't the first of their kind to arrive in China: they were preceded centuries before by the survivors of the 'lost legion'.