S1E16 : Millennium Swordsman

While searching for an Innocence in Rome, Kanda encounters a mysterious swordsman named Victorio, whose power is unbelievably strong. When Kanda hasn't returned after a while, Komui asks Allen and Lenalee to go to Rome to investigate why Kanda is taking so long, thus they learn the history of Victorio and the Princess, whom he vowed to always fight for.