Complex sentence: After this transformation, he fights Sensui on even footing until he is then possessed by his anscestor Raizen to help control his new-found power and as a result Raizen, through Yusuke's body, kills Sensui which upsets Yusuke greatly.
Split into simpler sentences: After this transformation, he fights Sensui on even footing until he is then possessed by his anscestor Raizen to help control his new-found power. As a result, Raizen, through Yusuke's body, kills Sensui which upsets Yusuke greatly.