Charlemagne is considered the first Holy Roman Emperor by most experts. The Holy Roman Emperor (historically Romanorum Imperator "Emperor of the Romans") was the ruler of the Holy Roman Empire. From an autocracy in Carolingian times the title evolved into an elected monarchy chosen by the Prince electors. Until the Reformation, the Emperor elect (imperator electus) was required to be crowned by the Pope before assuming the imperial title. Charlemagne reached the height of his power in 800 when he was crowned Emperor of the Romans by Pope Leo III on Christmas Day at Old St. Peter's Basilica.
When Charlemagne was crowned in 800, his was styled as "most serene Augustus, crowned by God, great and pacific emperor, governing the Roman Empire," thus constituting the elements of "Holy" and "Roman" in the imperial title.
Overall, Charlemagne or Charles the Great, was king of the Franks between 768 and 814, and emperor of Western Europe from 800 until 814. He founded the Holy Roman Empire, strengthened European economic and political life, and promoted the cultural revival known as the Carolingian Renaissance. Charlemagne's rule greatly influenced Europe's push to create a unique civilization different from that of Rome or other ancient empires.