Complete the game with the Onimusha rank. Alternately, get the Bishamon sword in the Phantom Realm in Honnouji when Sam goes to the past. To find it, go to the save location at the foot of the stairs to the temple, then look to your upper left to find the hanging man. Approach him to be sent to the Phantom Realm. Collect eight Red Orbs to unlock the chest that contains the Bishamon sword.