Near the end of Charlemagne 's reign and throughout the reigns of his sons and grandsons , a string of Viking raids began , leading to a Scandinavian conquest and settlement of the region now known as Normandy . Near the end of Charlemagne 's reign ( and throughout the reigns of his sons and grandsons ) , a string of Norse raids began , culminating in a gradual Scandinavian conquest and settlement of the region now known as Normandy .