Morgul-knives

The cursed knives used by the Nazgûl had strange and terrible properties, and the wounds they dealt could do far more than injure their victims. If a mortal's heart were to be pierced by a Morgul-knife, or even if a shard of a Nazgûl's blade should find its way to the heart, then that mortal would become enslaved, and transform over time into a lesser wraith. The enchanted blade of a Morgul-knife was able to fade away into the air if it was damaged or taken from its owner, but the evil magics inscribed on its hilt would still retain some of their power.