The Classical era Dark Age @1600-1,000 BC was caused by the eruption of Thera.
The Dark Ages that more people are familiar with was bracketed by the plague epidemic onsets of 536 and 1347. Numerous volcano eruptions and Earthquakes set the stage and preconditions for the initial outbreak of plague. You can go to the original hard copy sources or just use wikileaks but the pre-germ theories of these two Dark Ages have been disproven.
As always the case it was several factors, and yes the Thera eruption was a big deal at the time, disrupted and severely weakened several city states, including the Mycenaeans, around the Aegean and trade routes, may even have been the cause of the Etruscans migration from Turkey to the Italian peninsula. There was also the mass invasions of the 'Sea Peoples', probably several clans and tribes sweeping down by land and sea apparently from the Black Sea regions. Very short blurb here:
The Invasion of the Sea Peoples (1200 BC) A... ;
Their aggregate effect was probably worse relative to the population levels then than the Huns and Mongols. Remains to be seen who or what in turn pushed them south, maybe a revival of the Aryan cultures in the far north; recent ruins found in Russia seem to indicate a city older than the Aryan invasions of India, thought to be a city in the Aryan homelands last I read a couple years ago, whoever they were. Egypt was also in one of its periods of weakness.
As for the 'Dark Age' after the fall of Rome the population of the Med region had been in major decline for many years by the time of the 'barbarian invasions', while the population levels in northern Europe were taking a big long term leap, due to a warming spell; the center of power began gravitating there and the Med region left to decline. Many of the barbarians adopted much of the Roman culture and organizational methods, best they could anyways. It wasn't until after the 12th century that Christianity effectively moderated pagan brutalism in most of northern Europe, and even then it never entirely disappeared among many of the Germanic tribes and east. Witch burning was a pagan practice, and still a popular superstition among the peasantry and rural nobility. there is even a rough geographical component to the witch burnings and other reversions to pagan brutalism. Medieval law listed other crimes under the witchcraft heading, like poisoning and other forms of murder, so the term shouldn't be taken too literally, and in any case there were relatively few such hysterias, despite the rabid anti-Catholic propaganda spewed out by the Protestants during the Reformation and after, not to mention a few Protestants resorted to it themselves when the occasion presented itself.
Examinations of the records indicate around 10,000 to 15,000 such sentences over a 900 year period, probably way way down in numbers from the pagan eras that preceded 800 A.D. in northern regions; many pagans were into human sacrifices throughout most of human history. If one were to believe the propaganda gibberish from the Reformation and Enlightenment Europe would have a population of around 5 people; the numbers claimed are just absurd.