A sword that stores the identity of the wielder. To transfuse the soul to the blade, one must first stab himself through the heart. The person will not die, but lose the freedom of the soul upon real death.
Not every prophecy needs to be meaningful to effect a game. In the Lord Dunsany play, The Golden Doom, a child's scrawl has an entire kingdom struggling to puzzle out what sinister prophecy it portends.