Obtaining Kurosawa

There are a total of 5 Kurosawa keys, each obtained in a different way. The first key is awarded for completing the Land of Iron arc. The second key can be purchased at a secret bar for 50,000 ryo. The bar becomes available after completing the questline related to Mercenaries in your village. The third key drops from the boss Mifune in the Challenges mode. The fourth key can be found in deserter crates located in the Land of Iron. The fifth key drops from a deserter captain's pouch. Such pouches can be obtained at points of interest containing deserter captains.

After collecting all five keys, you must find the abandoned castle of Kaito Musashi, located in the south of the Land of Iron. The castle is a secret location and is not marked on the map. Its location must be discovered independently.

Inside the castle there is a gate behind which the Kurosawa sword is stored. To open it, you must insert all five Kurosawa keys into the gate. After installing the last key, the gate opens, allowing you to take the legendary sword.