Anthropology

Vikings pulled out their purses and became traders and their trade routes were extensive, stretching from eastern Canada to Afghanistan. When they saw easy pickings and turned into raiders, they took people as slaves. Slaves became part of the household and intermarriage happened often if the slaves weren't redeemed for cash before the Vikings moved on.

What killed the whole enterprise off was Christianity, Vikings found Christians would only trade with other Christians and they had quern stones to unload, preferably for silver. Never mind how prized those grain grinders were, the must-have appliance in every household, they found ports closed to them. So they converted and the church put a stop to all their fun, retribution for all the sacked monasteries and burned libraries.