|Dagger of Null Magic|
This broad-bladed Orichalcum bronze dagger glows black under the effects of a detect magic spell because it exudes anti-magic. The blade reflects light amazingly well. A few of these weapons were made by Ionian smiths as tests before finalizing the arcane transformation needed to create the Scutum Solis for the Legio Solmurus. As such, they feel somehow unfinished.
When the blade or pommel touches a person, all spells and magic items on that person cease to function for the duration of contact. The wielder can't cast spells while holding the dagger, and all equipped magic items have their magic suppressed. The wielder gains advantage on saving throws against magic. If the wielder desires after a successful attack, the dagger will remain stuck to the target, repressing all magical effects on their person and preventing them from casting of magic spells until they remove it and drop it using a bonus or move action.
As a weapon, it definitely has issues. The bronze blade doesn't hold an edge, so the dagger suffers a -1 penalty to damage. The broad blade makes it challenging to throw, imposing disadvantage on ranged attack rolls.
As a tool, it effectively prevents magical effects. In testing it suppressed arcane lock effects and magical traps like a charm. When plunged into a wall of fire, it suppressed the flames in a 5 foot radius. The effects seem to last indefinitely, so it can suppress a permanent effect for as long as it remains in direct contact.
Designer's Note: I want to use this as a trap, where the party finds it suppressing a nasty magic effect. The curious party member picks it up and gets bombed with the suddenly-active magical firestorm just as their magic equipment mysteriously stops working. This would work in rod form, but Ionian craftspeople generally have a hard time making an aesthetically pleasing plain metal stick.