It really is a good adaptation of history into a piece of fiction; I can't tell where the authentic history ends and your fantasy ideas begin. You kinda need that intro backstory though, since it has already does an effective job of drawing in at least three people so far. I was thinking that he is working as a (substitute?) history teacher in modern times, and the first chapter is him retelling that story of Bran to a bunch of primary (or maybe middle) school kids, who obviously don't believe it. That would be how I would introduce the rest of the story, which is whatever you were thinking.