Children of Dune

Muad’Dib has become an old man damaged by forced overdoses of spice essence and dependent on an assistant; he is rousing the populace against the priestly apparatus and its ruler — his sister Alia, who has since lost the battle with the memory personalities she contains, and is possessed by the persona of her grandfather and Atreides enemy, Baron Harkonnen.

Despite numerous enemies, Muad'Dib's children Leto and Ghanima survive concerted attempts to eliminate them. Leto undertakes a transformation by allowing sandtrout to bond to the surface of his body, making him immensely strong and fast and beginning his transformation into a human-sandworm hybrid. The subsequent deaths of Paul and Alia lead to the virtually immortal Leto grasping control of the Known Universe.

Over and over, Herbert shows how his characters' triumphs contain the seeds of their own destruction, and how their personalities and ideals keep them on the track of destruction, even if prescient vision proves to them how they are doomed. Frank Herbert said later in life that he conceived all three of the first Dune books as a single story from the start, and that he simply produced that one complete tale in three separate volumes.