How to unlock:
Get all Shinobi Moji, and then redo Mission 05. When you descend into the tunnels, walk forward, and turn left. If you haven't gotten all the moji, you will find that there is a small cave with nothing special in it. If you have, you will notice a hallway leading further inward. Go inside and claim the blade. This will subsequently end the mission.
Unlocks the final ability for the Stonerender. You have to use Blood Crystals to upgrade the Stonerender to the Moonlight Blade.