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I think the question you are really asking is, is the backstory the story you want to tell, or is it simply a fable on which the real story is based. None of us can answer that for you. If I had to...
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#2: Initial revision
I think the question you are really asking is, is the backstory the story you want to tell, or is it simply a fable on which the real story is based. None of us can answer that for you. If I had to guess, though, it sounds from the way you ask the question like you regard it as a fable that sets up the story you want to tell. So then the question becomes, when does the reader of the main story want to know the fable? The general rule of backstory is you only tell as much of it as you need to and only when the reader can't go on without it. On the other hand, because a fable is a story in its own right, and can be told very briefly, you can lead with it. A good example of this is the way Alan Garner tells the Legend of Alderley in _The Weirdstone of Brisingamen_. The thing about a legend, though, it that it is not a character piece. It is a kind of myth. It can be told briefly because our sympathy is not with any one character, but with some mythic hope or peril. Thus the Legend of Alderley is about a company of pure knights in enchanted sleep under a hill until the time when they are needed to save England from final corruption. Your backstory seems to have the elements of legend, so this same approach seems workable.