Complex sentence: He works with the Satsujinzoku to kidnap Eigetsu Tou and Kōrin (whom they had mistaken for Shūrei); believing that the group is under his grandfather's control, he attempts to shift their allegiance of their leader, Meishō, and gain their support for himself.
Split into simpler sentences: He works with the Satsujinzoku to kidnap Eigetsu Tou and Kōrin, whom they had mistaken for Shūrei. Believing that the group is under his grandfather's control, he attempts to shift their allegiance of their leader, Meishō, and gain their support for himself.