^Actually bleeding out from a stab wound can be a relatively slow death, depending on the wound.
Katana's Soultaker sword, along with its non-powered twin, was forged in the 14th century by Muramasa, whose swords were said to be cursed and make those who were evil commit evil acts. It sometimes takes the souls of those it kills, storing them inside the sword, where they can engage in limited communication with whoever wields it. These souls can be reincarnated by the means of a sacred ritual, under which they serve their summoner, even if it is against their will.
Admittedly, the whole "make those who are evil commit evil acts" thing does sound pretty redundant.