Magic items that you actually come up with and use in game are even cooler than the ones you just theorize about.
Show me yours, I’ll go first.
Athame of Eldritch Channeling : When you deal damage by wielding this weapon (hand, close) you my lose one spell you have prepared that could reasonably target that same enemy. The spell effects that enemy alone even if it would normally effect multiple targets.
This one was actually cooked up by one of my players and made with Ritual. We hammered out the exact mechanics six weeks later.