Lothaire Bluteau plays the "emperor"("king" in reality) Charles "the Bald", one of Charlemagne's grandsons. Lothaire is not a common name, not even in the french-speaking world. Strangely enough, Charles the Bald had two brothers: Louis the German who held the title of Emperor and Lothaire II who lost his possessions to his allied brothers.
(at around 4 minutes) The real emperor in Paris and grandson of Charlemange during the Viking sacking was Charles II, known as Charles the Bald. Despite this fact, he is portrayed here by Lothaire Bluteau with a full head of hair.