What's in a Katana

The Obsidian Goddess tells Musashi that she will be going back to sleep inside of him and no matter what foes he faces, he must not die. Because he is her vessel and because Jisai died before they were able to finish what they started. Musashi wonders what that has to do with Jisai, but the Goddess doesn't have time to explain and decides to show Musashi how she fights in the material world.