Otto III, Holy Roman Emperor

Otto III became King of Germany at three in 983, after his father Otto II died campaigning in southern Italy. His cousin Henry II tried to seize the throne in 984, failed to win the aristocracy, and stepped back. Otto's mother Theophanu ruled as regent until 991, then his grandmother Adelaide of Italy until 994. In 996 he marched south to claim the imperial crown and crush Crescentius II's Roman revolt, installing his cousin as Pope Gregory V. Crescentius rebelled again after Otto left, deposing Gregory and installing a rival pope; Otto returned in 998, reinstalled Gregory, and executed both Crescentius and the pretender. When Gregory died in 999, Otto installed Sylvester II. The Slavs had seized all imperial lands east of the Elbe after his father's death, and Otto never won them back, though he pushed Christianity east by crowning Stephen I as Hungary's first Christian king in 1000. Rome rose against him again in 1001, driving him out. He was marching back to retake the city when fever struck him down in January 1002 at Castle Paterno, leaving no heir and pitching the empire into chaos.