Buso Renkin, Vol. 6: A New Mission

Kazuki and Tokiko find themselves face to face with the legendary traitor of the Alchemist Warriors. The traitor, Victor, has a black kakugane for a heart, giving him the ability to drain energy from any humans in the area and increase his own strength. Then Kazuki discovers that he too has a black kakugane inside him. A fierce battle ensues, pitting the two owners of the rare black kakugane against one another!