Thanks for the tip, but that's not quite what I am getting at.
I don't know if you have seen the film Fargo, or the TV series it is based on, but both start with a line saying 'Based on a true story' when both are works of fiction. Fargo could be true however, because the events - although outlandish - could have actually happen.
What I am getting at here is a fantasy or science fiction story like The Lord of the Rings or Star Wars starting with the line 'Based on a true story', which obviously couldn't have happened because Orcs and Jedi etc. don't exist.
I'm just wondering if any fantasy or science fiction author has had the brass to start their story that way.