Complex sentence: In 857 the Holy Roman Emperor Charles the Bald ordered Horik to give up his lands between the Eider River and the North Sea what was then called North Frisia in retaliation for viking raids on Paris and the Loire Valley.
Split into simpler sentences: In 857 the Holy Roman Emperor Charles the Bald ordered Horik to give up his lands between the Eider River and the North Sea what was then called North Frisia. In retaliation for viking raids on Paris and the Loire Valley.