Mugen no Juunin: Immortal

Manji, a notorious swordsman in feudal Japan, earned the nickname "Hundred Man Killer" due to the one hundred innocent men he murdered. Even more dreadful than his sinister reputation, though, is the fact that he is everlasting. This was done by the 800-year-old nun Yaobikuni, who implanted bloodworms in Manji's body that could treat practically any wound. Manji decides to kill 1,000 bad persons in order to atone for his crimes. Yaobikuni accepts his request and promises to lift his curse of immortality if he is successful. Soon after making this pledge, Manji runs into 16-year-old Rin Asano, who asks for Manji's help in killing the people who killed her parents. Manji initially hesitates before turning down Rin's pleading request. However, Manji changes his mind and offers to look out for Rin for the next four years because of her obvious weakness. With this alliance firmly established, the two start off on a treacherous trip filled with bloodshed, retribution, and redemption in order to each achieve their own personal goals.