Many of those accomplishments, of course, were built on a foundation of decades of war that killed thousands. We've all heard of mass Christianizations by rulers of large sections of the populace, but haven't talked enough about mass slaughters. The two often went hand-in-hand.
Charlemagne spent decades focused on the Saxons to his east, wanting control over their politics and their souls. In 772 he destroyed the Irminsul, an important pagan symbol, and proclaimed rulership over much of Saxon territory. Ten years later, a rebellion rose up among the Saxons against Frankish forces at Süntel in Lower Saxony, joined by the Frisians. The Saxons prevailed, killing envoys of Charlemagne, four counts, and 20 additional noblemen.
When he heard this, the Lord King Charles rushed to the place with all the Franks that he could gather on short notice and advanced to where the Aller flows into the Weser. Then all the Saxons came together again, submitted to the authority of the Lord King, and surrendered the evildoers who were chiefly responsible for this revolt to be put to death—four thousand and five hundred of them. This sentence was carried out. Widukind was not among them since he had fled to Nordmannia [Denmark]. When he had finished this business, the Lord King returned to Francia. [Royal Frankish Annals]
The execution of the 4500 is called the Massacre of Verden. Some annals claim the 4500 were given a choice of baptism or execution, as the woodcut above displays. The instigator was Widukind, who continued being a thorn in Charlemagne's side over the next few bloody years until his final defeat and acceptance of baptism, along with the Frisians who had joined Widukind's Saxons.
Once Charlemagne subjugated a nation, forcing Christianity upon them, he would send missionaries. He would also establish a set of laws for them. In this particular case, the Frisians also asked for their own bishop. Charlemagne sent Ludger, later called "The Apostle of Saxony." He also created the Lex Frisionum, their own set of laws, which I'll dig into next.