Leto II and The Sandworms
In Children of Dune, Leto II initially avoids the high concentrations of melange which had given his father Paul complete prescience, and had precipitated his aunt Alia's descent into Abomination. As part of a plot set in motion by Jessica, Leto is eventually kidnapped and forced to consume large amounts of the spice; this "supersaturation" allows him to merge with sandtrout and ultimately become a human-sandworm hybrid.
Over 3,500 years the bulk of Leto's body is gradually transformed into that of a worm, and he is elevated to the role of the pharaonic God Emperor of the universe. In God Emperor of Dune he allows himself to be assassinated by Atreides descendant Siona Atreides and a ghola of Duncan Idaho. Cast into the Idaho River, his worm body separates into its component sandtrout, which immediately begin to undo the terraforming of Arrakis. Each one, according to Leto, carries in it a tiny pearl of his consciousness, trapped forever in an unending prescient dream. With the increased amount of neural ganglia and human-like adaptiveness the worms become too irritable to ride, but are also finally able to be transplanted to a variety of worlds across the universe. Over the next 1500 years Arrakis, now called Rakis, is returned to a desert and the worms thrive once more.
Bene Gesserit Mother Superior Taraza becomes aware in Heretics of Dune that humanity is being limited by the prescient dream of Leto, and controlled by him through his worm remnants. She engineers the destruction of Rakis by the Honored Matres to free humanity, leaving one remaining worm to start the cycle anew. Taraza is killed; her successor Darwi Odrade takes the worm to Chapterhouse. She submerges it in a spice bath to generate sandtrout, with the goal of populating Chapterhouse, and later other planets, with new worms and infinite potential for gathering spice.
The implication within Herbert's last two Dune novels of the 'prescient hold' that the residual ganglia within the Sandworms (and presumably Sand Plankton and Sandtrout) within the ecosphere on Dune, raises an important concept - the pre-Leto Sandworms also possessed ganglia, and therefore also must therefore have exercised a 'prescient hold' on the Universe in general. Being essentially animals, and non-sentient, this hold would then have expressed itself in the form of randomness, violent events, and even reproductively driven cycles within the lives of the Sandworms. Herbert did not expand on this concept in any of his works (the Dune Sextet, or Eye), and it remains an item of speculation on the part of both students of literary science fiction, as well as fans of the work.
Read more about this topic: Sandworm (Dune)