What is the main conflict in Children of Dune by Frank Herbert?

Children of Dune concludes the three-book story of Paul-Muad'Dib Atreides' life. His amazing young son, Leto II, helped by twin daughter Ghanima, surpasses Paul in finding the inner balance and unity of time and space required to lead humanity into a 4,000-year Golden Path of peace and prosperity. Paul's sister, Alia, exposed to the spice melange before birth, succumbs to the possession that she long fights, but which the twins channel adequately.