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.
When running a new party, get around the question of plot density by having the first discovery to be a deck of fate, or whatever you call it. If you use a real deck, you should definitely stack it to have the desired outcome, which is to curse the party to attract weird things to happen and involve them. This should not be told to the players at the time, so that they won't just up and try to get themselves 'decursed'. Though quite why they would want to is beyond me.