Sins. Some people have many, others have few. Those who do have a darker soul than those around them, and vice/versa. Normally such a thing isn't able to be seen. Why would you want it to anyway? [i]Well... I have an interesting proposition for you sinners and saints alike...[/i]:

 The sinner's sword... Unlimited power, if used correctly. Depending on your soul's weight, the sinner's sword can give you all the power you want. Power to smite evil, or... become the most powerful demon in existence. Of course, that's up to you.

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So, basically, this is a battle of good versus evil. Heaven has decided to judge humanity, and as such, has left a test behind and shut their gates. There is a single weapon from before human creation that has been bestowed upon humanity to do with what they will. But, heaven has told it's messengers to guide the blade to a shrine, where it should be placed, if humanity were to give up such a treasure. If they do, heaven opens back up to mankind. If not... well, the rest is self-explanatory. Those who have pure black souls will become powerful demons, and seek the blade to use it to gain power. Those who have pure white souls gain phenomenal holy power to use against the demons.

The sword of sins absorbs the weight of the sins others have committed, and converts it into power for the wielder to use. In the wrong hands, it absorbs an entire soul, and gives it's power to the demon. But in the right hands, It can purify even the darkest demon, if such a fiend is defeated. Your goal? Obtain the sword and lead humanity to a path of righteousness, or, become it's ruler.

(yeah yeah yeah, kind of over-used the holy/unholy dynamic, but I think this will be kind of fun. I already have a plot ready. I just need players. Oh, and the setting is modern-day.)