Weapon Formula

Is there a plugin that allows weapons to use the same damage formulas as skills? So like if I wanted to make a weapon that did more damage based on how much HP the user has for instance.

You mean, the weapon would use different "normal attack" formula?

Yeah, that's basically the same as asking "unique weapon normal attack / skill"

Thats what I mean, sort of like how some of the weapons in some of the final fantasy games work.

There are a few ways. The easiest is the Weaponskill plugin that comes prepackaged with MV. It simply lets you connect a different skill to your weapon. The other ways also involve plugins. Both Yanfly and Himeworks (which is most readily available) has options to choose from. Pick what makes sense for you.

As mentioned above, Yanfly has a plugin (Sadly can't remember the exact name right now) that allows you to adjust the settings of items by placing parameters in it's note tag.
I'm sure someone here can link the direct plugin, but I think it was "Item Core".
There are some examples as to how to accomplish this, and a solid description on the plugin page itself.