Sea Dragon (1990)

In the 10th century, Jestyn, a young orphan with a Celtic father and Saxon mother, is captured by Vikings and sold as a slave in Dublin; his master, Thormod, frees him for good service and Jestyn sails to Denmark on one of his longships. On returning to Ireland, he finds Thormod's father murdered. Jestyn and Thormod swear to avenge him, and the search for the killers leads them to the Baltic and Kiev. In Constantinople, Jestyn, still searching for his roots and his identity, discovers a totally unknown Byzantine world. Drawn in, he remains there.