Long has the question of truth been pondered. Long has this been a folly few recognise in time. A killer, a soldier even with a sword is one thing. But a killer who thinks he has found a definite truth, which he decides to fight for, is the most dangerous man in the world.
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.