“ If you want to add a 'supernatural' touch to your campaign, define the rules of magic and the universe. Make your players comfortable with those rules. Then your new supernatural creatures must then break those rules.”
“ A village where everyone goes blind sometime during puberty. What could be causing this? Perhaps something in their diet. Or some malignant gene. How would an entire village where only young childen can see function?”
“ In the middle of an unimportant combat with some bandits a burst of wild magic transforms all of the PCs and their opponents in to random animals and monsters. They retain their intelligence (though not, of course, the ability of speech). They can either carry on the combat in their new forms, panic, or otherwise react how they see best. After about an hour, they return, unharmed, to their normal form.”
