The Witch's dagger was an artefact belonging to The Witch. it was made up of a valuable material called Amesite.
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When a person first touches the dagger, it gives a burning feeling at first, and then it builds. The person also gets strange dreams and a voice that tells orders the person, and if you do not, she will kill you. Whenever the blade is close to a withc's mark, it causes a reaction to the mark. When in the dream-like state, if the vicitm looks into the mirror, the witch would be able to find him.
Ultimately, it only kills the soul, and not the body.