From what I understand, your world has gone through cycles. So now the heroes of the past are reincarnated.
What if the invasion is by humans. In a previous 'cycle', they set out into space to explore now worlds. They got stranded somewhere on a strange planet. This planet altered them (magic, genetic, whatever). Now, after many, many years, they regrouped, repopulated this other planet and now go about to seek their origins. They insist they want to reclaim their old homeworld. They see the current population as 'usurpers'.