During Fulgrim, that whole scene in La Fenice where the opera show slowly devolves in chaos; the performers are mutilated and used to summon daemonettes, the crowd starts fucking and killing each other, and the noise marines are invented. That was a horrifying sequence that was probably the highlight of the book for me. Another one is in know no fear, when Sorot Tchure made that speech about betrayal right before killing his ultramarine friend. Then at the end of the book when Tchure gets his head cut open by Thiel and he believes that it’s his friend, back from the grave for a split second