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.
Blood Scythe
This weapon appears at first to be a staff, made of a shiny obsidian-like material. When you grip the staff tightly, it will take your blood from you (1 armor-ignoring damage) and use it to quickly forge any polearm blade you desire. This blade seems to be made out a burning energy that glows a deep red, and will cut through even the toughest of armor as though it were freshly fallen snow. When the blade strikes an enemy, you may spend another HP to deal an additional +1d4 damage.
First wielded by the high cultist of the demon prince of fire, it was this very weapon that was used to slay the cultist’s dark master.
The Shield of Greed
A metal dragon coils around the outside of this shield. If you feed the dragon a gold coin (placed in the mouth) any and all damage to the shield is repaired.
The protector’s shield
This shield is wider than the average shield, with wings of metal extending to the left or the right. They seem to feather out when an ally close by is in trouble.
When you defend someone else, you can hold 1 even on a 6-.
Potion of Deeds…
One use, on imbibing allows character to choose any one move from any class and make it if warranted in the fiction.
The Smuggler’s Bag: Cloaked with an illusion of emptiness, unless you speak the proper command word when you open it. Also, when you find it, probably has a fair amount of illicit contraband of various sorts in it.