Complex sentence: Umbridge creates The Inquisitorial Squad, which is comprised entirely of Slytherins and rewards some students for reporting on others and sanctions them to act as enforcers of Umbridge's rules.
Split into simpler sentences: Umbridge creates the Inquisitorial Squad, which is comprised entirely of Slytherins. It rewards some students for reporting on others and sanctions them to act as enforcers of Umbridge's rules.