Start of the Dark Ages - Then, Not Now.

The Fall of the Roman Empire - Perhaps.

THIS ARTICLE suggests that a volcanic eruption in Central America was the cause of plague in Constantinople, widespread famine and barbarian invasions of the Roman Empire. 

The basis for the popular article is THIS PAPER. The main purpose of the paper is to date accurately when Ilopango (in El Salvador) erupted. You need access to read the original but I notice, from the abstract, that 100,000 are estimated to have died in Central America, but this is not mentioned in the popular article, but then, it is about the Roman Empire.