Creative illustration. History of Rome Charlemagne also known as Charles the Great, was the King of the Franks from 768, the King of Italy from 774, and from 800 the first emperor in western Europe since the collapse of the Western Roman Empire. At Mass, on Christmas Day 25 December, when Charlemagne knelt at the altar to pray, the Pope crowned him Imperator Romanorum . Emperor of the Romans. in Saint Peter's Basilica. In so doing, the Pope was effectively nullifying the legitimacy of Empress Irene of Constantinople:.