From StrategyWiki, the video game walkthrough and strategy guide wiki
Play the original Ninja Gaiden games
You have to unlock them in sequence (e.g. you cannot get Ninja Gaiden 2 until you unlock Ninja Gaiden).
- Ninja Gaiden: gather a total of 50 Golden Scarabs and give them to Muramasa. You will receive a Ninja Gaiden disk. Note that you do not have to collect all 50 in one play through; you can collect them across consecutive games.
- Ninja Gaiden 2: after unlocking Ninja Gaiden, go to the Clock Tower Plaza of Tairon City. Get on the ledge above the multicolored door and hit the face of the Clock Tower with your bow. A treasure chest will appear on the building opposite of Ryu. Jump down to the bottom, then go up via the doorway. The chest contains the Ninja Giaden 2 disk.
- Ninja Gaiden 3: go to the Chapter 10's Peristyle Passage. Go to one of the broken pillars and you will find the Ninja Gaiden 3 disc.