The death of all other sandworms as a result of the terraforming of Arrakis, and his control of the remaining supply of the all-important spice, has allowed him to keep civilization under his complete command.

Leto has been physically transformed into a worm, retaining only his human face and arms, and though he is now seemingly immortal and invulnerable to harm, he is prone to instinct-driven bouts of violence when provoked to anger.

He has rendered the human population into a state of trans-galactic stagnation; space travel is non-existent to most people in his empire, which he has deliberately kept to a near-medieval level of technological sophistication.

The Ixians send a new ambassador named Hwi Noree to serve Leto, and though he realizes that she has been specifically designed and trained to ensnare him, he cannot resist falling in love with her.

When Duncan falls in love and copulates with Hwi, Moneo sends him and Siona out to Tuono Village, an outcropping along the Royal Procession road, to keep them safe from Leto's wrath.

As Leto's wedding procession moves across a high bridge over a river, Siona's associate Nayla destroys the support beams with a lasgun.

Having begun millennia before with the union of Leto's twin sister Ghanima and Farad'n of House Corrino, Siona is the finished result, and she and her descendants will retain this ability.

After revealing the location of his secret spice hoard, Leto dies, leaving Duncan and Siona to face the task of managing the empire.

Leto II personally explores the emergent effects of civilization, noting that most hierarchical structures are remnants of evolutionary urges toward safety.