What's that?
An axe-age, a wolf-age when the hand of kin is turned against kin.
Brother war with brother and father fights son.
The Fenris wolf breaks his bonds and devours the moon, and bane-wounds the sun.
No day but only a white haze where the sun can barely be made out.
Biting cold winds blow snow from every direction,
Three such winters follow one after the other with no summer in between and the snow does not melt.
Most of mankind perish.
The Fimbulwinter marks the start of Ragnarok, the end of the world in the Norse beliefs. Lately, we've come to realize that this is actually a folk memory of the years 536-540, when the majority of the people in Scandinavia died and the society were violently transformed. It was fairly apocalyptic, as far as can be made out. 70-90 % of farms and settlements get abandoned, trees have no growth rings for four years, craft and specialization collapses. The only traces human activity that increases is sacrifices, the numbers go into overdrive. When it all settled down, the result was the Norse. Actually post-apocalyptic warriors.
Cite, needs google translate
Cite.
Britain is not far enough away from Scandinavia to be unaffected by something that can stop trees from growing for years over such a large area. Although if winds and climate did mean it got off lightly, and then got the plague, it would explain a lot about why the Angles, Jutes and Saxons were so set on getting there.