How the continent got separated from the rest of humanity's. That they not only developed their own magic but spied on the rest to steal what they developed. What happened when the separation fell apart -- weather catastrophes, plagues, conquest (hmmm -- probably at least in part motivated by the catastrophic weather on the outside).
Why it fell apart? Hmmm -- could be just wore out. Could be it was tied to Royal Blood and the last heir died. Could be break down of the proper rituals, especially if those needed to be wide-spread. I consider the pros and cons of all of them.
Then I remember: it's backstory happening at least a few centuries before the story, and a major part of the story is piecing together what happened before. I don't need to decide, because they don't know!