What is an interesting magical item/weapon in your world?

What are its unique magical (or technological) properties?

One of mine would have to be 'Mercy'; an iron dagger that instantly heals the wounds it makes. (It's also known as the 'ironic dagger':) The dagger is named 'Mercy' both unironically and ironically. It's merciful because it, well, heals up the wounds it gives you, saving you from dying. But killing you is not the dagger's purpose, it's a psychological weapon that is used mainly as a torture device for interrogations. However, overusing the dagger on a victim over a short period of time could cause death from emaciation, since the blade uses the victim's bodily nutrients to heal the wounds it makes (the dagger works by accelerating the body's natural cellular regeneration process but relies on the body's nutrients to make new cells.)

If you were to be stabbed in the brain with this dagger, the wound would heal, but numerous psychological complications could occur... A major character in my world; an assassin named Ridge, gets stabbed in the brain by the dagger during an intense interrogation - as a result, he loses all memory of his past and is driven a little bit insane. His story arc centres around him rediscovering his past and fighting ability as an assassin... he ends up stealing the dagger at one point and it becomes his signature weapon. He uses it to defeat his opponents by stabbing them in the brain, the results of which could range from outright insanity, memory loss (as Ridge had experienced) or brain-dead unresponsiveness, depending on the angle the victim was stabbed. Ridge manipulates those with memory loss to do his bidding. The weapon can actually be fatal when stabbed at the heart in certain angles, blocking blood flow to the heart. You can also cut off small body parts with the dagger (like an ear or a finger) if you applied enough force with the blade; the remaining stump would heal over with skin, however.