Dorn and Fulgrim call out Curze for War Crimes. Curze is imprisoned on the Phalanx. He subsequently Batmans his way out ('Batman' is a verb now). Curze attempts to kill Dorn. Dorn doesn't fight back. He is only saved by Sigismund. The Dark King.
It is unclear how far Fulgrim is along in his particular journey at this point.
Curze is the only thing in the galaxy that Dorn is afraid of. Because there is no reason to his actions. Dorn starts to wonder what it would take for him to turn against Dad, and he begins to accept that his brothers might not be his brothers anymore. Becoming resolute they they aren't his brothers anymore in Deliverance Lost.
Saving Lorgar.3- During the Drop site massacre he fights Corax forcing him to flee?
Otherwise, I have nothing to add.