Everyone dies. But delaying that impending sentence is the holy grail. Science, scorcery, and wishful thinking do all in their power to bestow such delays. Be your deeds pure or evil, cheating death is neither easy nor reliable.
Wanderers beware, these gates open not,
Except to those only who the way have bought.
You may purchase the right to pass me by,
By each guessing my riddles. Care to try?
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.