"Reills. E'en the name makes me shudder. That there is unholy ground, cursed by most gods. Not even the demons and devils and the incarnations o' evil dare step foot there. Reills. You know me, I think money likes it in my pocket instead of wherever else it is. But, and I've heard rumors that say this, if theirs treasure on that ground, it ain't going in my pocket on pain o' death."
-Old Gerald, man in the pub.
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.