Still somewhat at a loss. So the character I'm thinking about would wield a magic katana handed down in her family. It would have been one used against mages during their persecution. As such it's one that would give an ordinary human the capability to compete with a mage. But I can't quite put my finger on how best it should do that. I'm leaning toward it having something to be with magic sense as an aura. But beyond that I'm having difficulty.
Here's the image and inspiration of this concept:
@Expolar
Here is an idea, it has two abilities, one the ability to 'shield' the wielder from magic, think of it as a Resistance bonus in an RPG, and twoit can fire sword beams in a projectile attack.
because who doesn't like beams?! Nah, The reason for this is that most mages would want to attack from afar, or midrange, so it gives the sworduser some reach.
No, depending on how powerful you want the thing, you could go a little further. If you want to make it super powerful, able to match with S class wizards, you could give a specific ability to counter magic, like devouring the magic, or reflecting the magic back. The second is self explanatory, but the first could go pretty deep, again, depending on how powerful you want it. It could literally syphon magic from who ever it cuts, or just whoever is in its general vicinity. It could 'eat' spells and utilize the magic itself, or give the wielder the stolen magical energy. Could even be sentient.
Just some stray thought to jumpstart your creativity.