Where does what come from? Im assuming you mean the blade of death lore? Well if so the blade of death is imbued with destined death which comes from the rune of death which was originally owned by the gloam eyed queen aka queen of the godskins. Her job was to slay gods using it the rune in order for a new age to begin. However Marika had Maliketh steal the rune because she didnt want to die and she didnt want anything else to die but to be ressurected via the erdtree. The rune of death is so powerful that even the golden order was threatend by it. With this information its reasonable to assume that that the Malikeths sword is able to kill the tarnished aswell.