AD 635-700s: The Islamic conquest of about half of the former Roman empire—including it’s breadbasket, Egypt & Libya.
With Muslim pirates also trolling the Mediterranean—the sophisticated Roman international sea trade routes—and land routes of the east and south, are cut off and trade comes to an end. With trade cut off to the North, the economy shrivels....and with it culture and education.
The Muslim conquests, as much or more than any other factor, brought in the Dark Ages to Europe....
From what I have read the impact of the 536AD volcano eruption (and this possible impact of Halley’s Comet debris) affected the weather which influenced Mohammed and his travels. History might have turned out much differently if these two events had not occurred.
The Roman empire was already in trouble. Started with movement of the tribes in the late 300s. Visigoths, Ostrogoths, Vandals, Huns, etc. In 451 the Huns under Attila sacked Rome after he had failed to conquer Constantinople, in 455 the Vandals sacked Rome, after sweaping up from North Africa which they had conquered and occupied some decades before. So Rome was already a lame duck when the Muslims came along.