Attila the Hun in 441AD-450AD - with the death of his brother - achieves rule over all Hunnic lands, and achieves his infamous reputation.
Julian the Apostate was the last pagan emperor of Rome, who was the last emperor to try and end Christianity - but the game was up.
Buddhism's arrival in China forever changes Chinese culture, and Halley's Comet playing a unique role in the First Jewish-Roman War.