It depends on when you make Mirage Mirror into a copy of the Saga. 

If you copy it in your main phase or later, then nothing much will happen and Mirage Mirror will just go back to normal when the turn ends. 

If you copy the Saga before your first main phase, then it will get a lore counter when your main phase begins and the appropriate chapter ability will trigger. If that happens to be the final chapter ability of the card it’s copying, then you’ll sacrifice it once that trigger resolves.