You were sneaking your way into Ashina castle to save the young lord. A little girl by the name kanna

(Shhhhhhh >:3

She had the power to give whoever she chose immortality. And she her shinobi. You. She was protected by the ruler of the castle wanting the power for himself. Genichro Ashina. You get to the top of the castle where they kept the young lord but before you got into the room you were thrown back. "You shouldn't have come here shinobi." Genichro said drawing his sword. After a intense battle you had him on the ropes. But before you finally killed him a shuriken was thrown into your arm causing you to shout in pain and loose your focus. This moment allowed Genichro to slice your (left/right) arm off and the sudden trauma causes you to stumble off the building and into the moat. Awakening who knows when later you see a old man with one arm carving something. And you and him were surrounded by Buddha statues

Before you asked him what happened you noticed something on your arm. It was a prosthetic

You?