"Here lies a peasant child scarred by the heat of battle.
Fighting not for glory but because he just wanted to live,
His despairation led him to victory."
~ Carved on the ornate stone doors leading to the large tomb.
Historically, most tombs known to have grave goods in them were robbed soon after the dead person was laid to rest. So why has the tomb that your group of adventurers broken into not already suffered this fate?
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.