Viking mythology says that there will be a long winter before Ragnarök, the end of the world. Long as three normal winters but with no summer in-between. After that winter there would be war. Maybe this Fimbulwinter really happened and inspired the mythology? In the year 536 there was a volcano eruption covering the sky and blocking the sun (and probably another eruption 540). Harsh years followed and crops failed. A lot of people died, not just in the Nordics. We’re just learning more about this event.
The Telegraph: Did American volcanos trigger the fall of the Roman Empire?