The Franks were sort of the odds on favor to be the up and coming people in Europe. The relationship changed both entities and changed Christianity also. Charlemagne was when he starts out a king of the Franks and by the time he ends, he's the emperor of a renovated Roman Empire.
This show picks up where Dan's Thor's Angels show left off. In the early Middle Ages Pagan Germanic-language speakers like the Vikings are a dying breed. Many of their contemporaries wish they'd die faster.