The forth book in the Dune sequence. More than a millennium has passed since the first events recorded in Dune. One link only survives with that epic past: Leto Atreides, God Emperor of the galaxy. He alone it is who understands the future, who knows with a terrible certainty that the evolution of his race is at an end unless he can breed new qualities into the species. Yet he knows also that these new and vital forces can only prove themselves through the fateful process of his own destruction.
The"God Emperor" is a descendant of the original Paul Maud'Dib that has undergone a physical transformation, giving him (most of) the body of a Dune sandworm, while retaining his mind and prescient powers. The Universe is completely in his iron grip - even the farthest flung planets that think they are outside his sphere of control. The God Emperor has been slowly transforming into the Great Worm for the entire period between novels, and obviously is ~extremely~ long lived. The empire no longer faces threats external to the Atriedes clan. It is the offspring of the Emperor himself causing unrest. As the Emperor is nearly Omnipotent, he prefers not to interfere with the rebels, choosing instead to see if they can try anything which he hasn't already seen in his multi-millennial life.
