I once read a book about a group of people who discovered a hidden temple city in a mountain range, where the Incas-the ancient civilisation of Peru- had fled to when the spanish invaded. When this group found the burial chamber of one of the inca high priests, they unleashed a powerful curse upon them all...
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.