Dragonbane starts off as just a weapon with a damage bonus versus dragons, but it gains properties corresponding to every type of adult dragon you have killed with it. So when you kill a red dragon with it, it now gives you fire resistance and/or does fire damage, then you kill a black dragon and it now gives you fire and disintegration resistance and/or does fire and disintegration damage, and so on.

This doesn’t have to be Dragonbane, the concept of an artifact that powers up by killing different monsters could apply to something else, but Dragonbane is a good example since dragons have a big diversity of obvious properties and resistances.

To keep it from becoming overpowered, it could stack up to 2 or 3 properties from the last 2 or 3 unique types of dragons killed so long as neither property is something powerful like reflection or magic resistance (which would clear all other properties).