What is the rarest katana?

Could you please elaborate on the topic of the rarest katana? Are you referring to a specific historical piece, a modern collectible, or perhaps a katana that is exceptionally rare due to its unique craftsmanship or provenance? Understanding the context of your question would help me provide a more accurate and informative answer. Additionally, are you interested in the rarity of the katana in terms of its scarcity, historical significance, or perhaps both?