'Add your own Fluff' D&D Magic Item: The 'Save v.s Death' Knife

  So this is the idea; A magic knife that anyone stabbed by it has to roll a Saving Throw v.s Death or die. However if the target succeeds in their roll, the wielder of the knife must then make the same Saving Throw or be the one that dies! You know, because the knife is hungry for souls.
  Anyone killed by the knife can only be brought to life following the destruction of the knife. (How the knife can be destroyed is up to whatever system your specific version of the game uses or the DM's own personal ideas.) Alternatively, you could use a Wish spell to get the soul out of the knife and then use whatever other spell (including Wish again) to bring the victim back to life.
  Its magical effect won't work on the undead, constructs or any other opponent who's already dead and/or soulless. Those will only receive the normal 1d4 damage. The knife will still try to take the wielder's soul in such a case, it hates being teased with the promise of a new soul to devour only to find there's nothing to take.
  I can see this object being used as an 'all-or-nothing' weapon against opponents that the player otherwise doesn't stand a chance against. Which is a gamble in itself since a tougher opponent will very likely have a better Saving Throw than the player. If it's used before being properly identified, it could also lead to some very surprising situations for your players...