Complex sentence: The Normans had come from the French province of Normandy, which was granted to the Vikings by the French king Charles the Simple and from then on these rebellious vassals took the name of their new found land.
Split into simpler sentences: The Normans had come from the French province of Normandy. The land was granted to Vikings by a previous French King Charles the Simple and from then these rebellious vassals took the name of their new found land.