Post Mons Badonicus? Or is that too early.That might work, but what time period would be the best to have the plague occur when no outside group can easily conquer Wales?
EDIT: Wow, that was a confusing sentence. What I mean is, what's the best time period for the plague to occur? In a large portion of the independent (if divided) Wales's history, it was in danger of being taken over by whoever controlled England at the time. When is the best moment in history to have the plague strike Wales?
Alternately, the time of Canute? The Welsh were his vassals so he might not see a need to conquer them. Alternately he might decide to strike while they are weak and bring them fully under his banner.