The Witch's dagger was an artefact belonging to the Witch known as Elizabeth Dexter. it was made up of a valuable material called Amnacite.
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, rthe witch would be able to find him.
Ultimately, it only kills the soul, and not the body.