That's one of those things that would wipe out the people, the animals and the plants.
According to extant British records of the time Brittany was pretty much wiped clean at the time, and there's a band across the Scandinavian peninsula that seems to have been treated similarly SEVERAL TIMES over the last few thousand years.
Those kind of speculations are really interesting. Could change our understanding of history.
What if no one had any particular interest in attacking Rome but some cataclysmic event set in motion some chain of events?
Only a relatively few volcanoes can cause that that. Laki or Krakatoa (which some attribute the 416 eruption to this year) might be able to do so.