Complex sentence: Riddle states he has grown strong on Ginny's fears and eventually possesses her, using her as a pawn to unlock the Chamber of Secrets, whence a basilisk is set free and petrifies several Hogwarts students.
Split into simpler sentences: Riddle states he has grown strong on Ginny's fears and eventually possesses her. He then uses her as a pawn to unlock the Chamber of Secrets, whence a basilisk is set free and petrifies several Hogwarts students.