Muramasa katana - the demon-killing swords
Here's an interesting thing -
http://en.rocketnews24.com/2016/09/23/amazing-exhibition-of-japans-legendary-cursed-katana-is-going-on-right-now【photos】/
Murumasa in this film is making the blade to kill demons - but you need to be careful with that sort of thing. Does the owner of a Muramasa become a demon himself? Oooh...