This sounds like it could be a cool (and rather daunting) campaign setting. It could take place in a a Thule where aforementioned events have occurred (as they so often do throughout history, for all peoples), perhaps implementing some of the rules from The Coming where they can be applied.
Dark Age would mean that Thulê was conquered by Christianity or some other foreing religion. The term "Dark Age" comes from the fact that we don't have any or very little records from the period of time, since church has destroyed the history.
We can thank scholar names Petrarch for this term for the post-roman centuries.