On this day 21 May in 996 Otto III was crowned Holy Roman Emperor in Rome. He was only 16 years old and was crowned by his cousin Pope Gregory V whom he himself installed as the pope less than a month prior to that. This is how things worked at the time.
Otto had already been crowned King of Germany in 983 at the age of only three. His mother Theophanu ruled as a regent, and after her death in 991, his grandmother Adelaide of Italy had that role until Otto became old enough to govern.
When Otto grew up he became very ambitious. He had to fight campaigns against Slavs and Vikings to protect German borders in the east and in the north. But in 996 he focused his attention on Italy where he took advantage of instability in Rome and asserted his control there.
Otto III planned to restore the ancient Roman Empire and made Rome the administrative capital of his Empire. He tried to revive the splendor of Rome by reviving elaborate customs. His power and prestige grew as he was able to control the Church through his cousin the pope.
This image from Otto III's Gospel Book made in 1000 displays the image of the universal ruler of Christendom he wanted to project. Various races of Europe, personified by women named Sclavinia, Germania, Gallia and Roma, are depicted bringing gifts to the Emperor!
But the Roman nobility saw him as a foreigner and was not happy about this German takeover of papacy as they had traditionally named the pope themselves from their ranks. In 997, a year after Otto III was crowned and left Rome, the Romans rebelled!
The revolt was led by a powerful Roman aristocrat Crescentius II who had declared himself Consul of Rome. The rebellious Romans were also supported by the Byzantines. They tried to install their own Antipope John XVI! But the rebellion failed when the Emperor returned in 998.
Otto III returned to Rome with vengeance and reclaimed the city from the Roman aristocracy. His cousin Gregory V, who had fled to Pavia, was reinstalled as the pope. The Antipope John XVI tried to flee but was captured by the Imperial troops and was brought before the pope.
This Antipope John XVI was badly mutilated by the Imperial troops who had cut off his nose and ears, cut out his tongue, broke his fingers and blinded him. But Pope Gregory V decided to spare his life and sent him to a monastery in Germany where he died three years later.
Meanwhile the leader of revolt Crescentius II fled to the Tomb of Hadrian which was the traditional stronghold of his Crescentii family. The Imperial troops besieged the fortress and Crescentius was executed. Such was the end of this man who claimed to be "Patricius Romanorum".
But the Romans would rebel against the Imperial rule several more times during the short reign of Otto III. He crushed these rebellions but died in 1002 aged 21, just as he was about to marry a Byzantine princess. He left no heir and the Imperial control over Italy weakened.
The sudden death of Otto III was a huge blow to his project of renewal of the empire of the Romans (renovatio imperii Romanorum). Over the 11th century, the popes would begin to consolidate their power after a series of conflicts with Emperors, limiting Imperial influence.
@germanshistory has done an amazing podcast on Emperor Otto III and these events and much more!