486-571 (and later)
Something major happened to Earth’s atmosphere in 535. We have reports from around the world of the sun being darkened or blotted out for more than a year, and evidence from tree rings and ice cores of an extreme cold spell. The culprit might have been dust thrown into the atmosphere by volcano or a comet. This on its own must have been bad news for the world’s population. But even more consequential was what happened starting seven years later. In 542, bubonic plague made an appearance in the Egyptian port of Pelusium, and rapidly spread around the Mediterranean, eventually reaching much of western Europe and Persia. (China seems to have gotten off more lightly.) It’s possible the epidemic had its origin among rodents in the east African Great Lakes region: disturbances to these populations after 535 may have contributed to the spread of plague, either up the Nile valley, or to trading towns on the Indian Ocean. Recent genetic evidence has confirmed that plague bacteria from this period are almost identical to those from the later, better known Black Death in the late Middle Ages. The plague struck repeatedly around west Eurasia for the next 200 years, before disappearing. The death toll must have been many tens of millions.
Major movements of peoples followed the plague in the sixth and seventh centuries. The Byzantine reconquest of most of the western Roman Empire, under Justinian, came undone as a new wave of Germanic barbarians, the Lombards, occupied Italy. The Anglo-Saxons expanded from the east of England to occupy most of present-day England. Slavs moved south to occupy most of the Balkans. And, most consequentially, Arabs under the banner of Islam occupied most of the Middle East and North Africa.