One Piece : Dark Swordsman

After Monkey D. Luffy became the king of pirates and Roronoa Zoro became the greatest swordsman in the world
The two companions separate, and each goes to live the last rest of his life
Meanwhile, Zoro meets a little boy
As he asks him to teach him how to use the sword in order to become a strong and well-known swordsman
And this is where the suspense begins