Eg. After this period, in the early 600's, Slavic tribes migrated south, all the way to Greece/Egean sea, but eventually were pushed back to current/modern areas....
So, these events contributed heavily to even modern borders and some events....
I know, there are some weird post-modernist movement to say 'dark ages were not that bad', but indeed, these were some of the darkest/harshest time in our recorded history....
The volcano being in Iceland, could explain on why Britain was one of the harshest hit areas by the dark ages....