Like I said in the previous answer they are random events. They will randomly pop up after a battle what I advise you do is capture all castles but one.(The best options are either the Water or Ice nation to be that last uncaptured castle. That way you can block them off from conquering more castles.)Then build up a strong team for 3 different castles and surround that single castle. You can spend the rest of the time battling and waiting for the crystals to appear.