Ep. 8: Rukis Abstieg in den Wahnsinn und die Entführung von Yui durch Ayato

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Due to his realization that he and his brothers may not be able to become "Adam", Ruki begins to act strangely to the extent where he confines Yui in a small room to forcefully suck her blood. But then he calms down and remembering when the Mukami were on the verge of death as a kid, a man appeared and save their life to fulfill the "promise" from him. As this happens, his brothers grow concerned for him and Yui as she becomes weaker. Ayato takes action abducting Yui to which Ruki offers no resistance much to the confusion of his brothers. (Source: Wikipedia)