It’s one of those mysteries of history for which no cause is known. According to written records, for a year to a year and half in AD 536, a thick, persistent dust veil darkened the skies between Europe and Asia Minor. It’s believed that it covered numerous other parts of the world though.
The cloud is known to have gone as far east as China, and even Mongolia, Siberia, Argentina, and Chile. The mysterious dust veil decreased temperatures and brought massive drought and food shortages. As this was right before the smallpox outbreak in Europe, as much as a third of Europeans were left dead.
In China it was even worse. The famine caused by the dust veil killed up to 80% of people. In Scandinavia, it could have been as high as 90%. A Syrian from that time wrote “the sun became dark and its darkness lasted for one and a half years... Each day it shone for about four hours and still this light was only a feeble shadow...the fruits did not ripen and the wine tasted like sour grapes."
There are many other accounts that confirm this mysterious cloud, but little answers. One account claimed it lasted from 24th March 536 to 24th June 537. Today its effects are seen on the rings of trees that have existed since then.