Instructions
Beat both characters' storylines twice. There are a total of three story arcs for both characters in the game. The third story arc unlocks once you forge the Oboro Muramasa; the first two arcs must be completed before the Muramasa is available to forge.
Create or find every sword in the game. There are dozens of unique katanas and other weapons in the game, some of which can only be used by one character or the other. You must collect all of them before the Muramasa becomes available.
Enter the forge screen after completing all of the requirements. The Oboro Muramasa will be available and can finally be created.