A Sword?

Yes. A sword. An amazingly evil sword that gets stronger everytime you kill someone with it. Well not every time but in incriments.

For example: After 10 kills it may get a +1 modifier. 25 kills, a +2 modifier. Maybe after 50 kills you get a +1 to strength while wielding it.

It's just a concept at the moment I have to lay everything out on paper to see how far apart the kill spread should be. I do know, however, that despite it being an evil sword anyone will be able to use it. They'll just become a little power hungry with every "enemy" felled in combat (or out of combat) and consequently a little more evil.
The sword may even hint that it doesn't like the wielders party members and thus grating more power if they're offed.