this hapens in the manga near the end of volume 38 . I'm not sure when its gonna happen in the anime but i can imagine soon. I don't wanna tell the whole story cuz i'm lazy but u can read the summery at http://furinkan.com/iycompanion/manga/. Ok, naraku learns of kagura's betrayal to him and tells her that he is going to set her free. he gives her her heart back and then stabs her. she has just enough energy to get away from his castle and into a feild where she waits for death. Sesshomaru comes and trys to revive her with Tenseiga but for unknown reasons it doesn't work. Inuyasha and the gang (including Kohaku) arive just in time to see her body disentigrate into the wind.