Leto Atreides II is a fictional character in the Dune universe, created by Frank Herbert. Leto is a central character in Children of Dune and is the central character of God Emperor of Dune. He is son to Paul Atreides and Chani.
Children of Dune
Leto is named for his paternal grandfather Duke Leto Atreides, who was killed in the Harkonnen / Imperial invasion of Arrakis. He is the second child of Paul to bear that name, the first having been killed by the Emperor's Sardaukar.
In Children of Dune, Leto and his twin sister Ghanima are nine years old. Because of the spice ingested by their mother, Leto and Ghanima are "pre-born", meaning that, as foetuses in their mother's womb, they were awakened to consciousness and to their genetic memories; thus, they are born as fully matured human beings in the bodies of infants. At the start of the novel, Leto is not prescient to the degree that Paul was, but he senses the test his father faced: to embrace a prescient vision of the universe is to set the universe on that path, a terrible responsibility that comes with terrible power. At the end of Dune Messiah, Paul forsook that responsibility by walking into the desert--his time as the Fremen messiah had shown him that he was not strong enough to be messiah/tyrant to the universe. Leto believes that he must face the same test.
At the same time, the Imperium Paul created is ruled by his sister Alia Atreides as regent. The horror of the pre-born, the reason the Bene Gesserit call them "abomination", is that they are easily possessed by the ego-memories of their ancestors. When Bene Gesserit awaken their 'other memories' in the ritual of the spice agony, they are adults with fully formed personalities, and can withstand the inner assault of their forebears; the pre-born have no such defense. Like Leto and Ghanima, Alia was pre-born, and she succumbs to the pressure under an intense dose of spice. Among her ancestors is the Baron Vladimir Harkonnen, still hungry for revenge against his enemies, the Atreides. Alia is possessed by him, and unconsciously turns against the Atreides empire, plotting to kill Leto and Ghanima and to tear down the Imperium in a bloody civil war.
Leto faces the test his father refused to take, and embraces prescience, its visions, its attendant power, and the terrible price it will extract: to follow his vision, Leto will become a sandworm and rule for thousands of years, effectively immortal until humanity evolves enough to kill him. By doing so, he will set the universe on "The Golden Path", a future in which humanity's survival is assured. Leto disappears into the desert following an assassination attempt by House Corrino, leaving Ghanima behind.
Independently, Leto and Ghanima both solve the problem of the pre-born. Leto constructs his own personality out of an executive of his ancestors; with all (the important ones) possessing him, none can possess him individually. As part of Leto's plan, Ghanima hypnotizes herself to believe that Leto was killed in the assassination; the intense mental discipline this demands builds a safe haven in Ghanima's mind for her own personality to safely develop. Would that Alia made the same discoveries.
At the end, following his test and his embracing of his vision, Leto returns to wrest the Imperium from Alia and take his rightful place as Emperor.
God Emperor of Dune
3,500 years have passed, and Leto is now almost fully transformed into a Sandworm. He is almost invulnerable to physical damage; his single weakness that he shares with the sandworms, an intense vulnerability to water, is a secret. "Leto's peace" has kept the universe quiet for that time, and the entirety of human society has become an audience for him. He is their emperor; he is their god. His all female army of Fish Speakers keeps order and acts as priestesses (according to Leto, a female army is a nurturing disciplinarian, while a male army is essentially predatory and always turns against its civilian support base in the absence of an enemy).
The old institutions, the Bene Gesserit, the Bene Tleilax, the Spacing Guild, the houses major and minor, the Landsraad, the technocrats of Ix, and CHOAM, have all faded from power in the face of Leto's hydraulic despotism: since he has absolute control of a single element on which the whole universe depends, he has the universe by the throat, and ruthlessly enforces his simplistic order.
Leto has taken over the Bene Gesserit's breeding program for himself, the same program that produced his father, the Kwisatz Haderach. His Golden Path is nearly assured now, and 3,500 years of rule has begun to bore him. The trap of prescience is an existence without surprises; Leto is also such a long-lived creature that he has seen it all, especially with the help of his other memory. There is nothing left for him to do but to ensure that humanity will survive. To accomplish this, he has spent the last three millennia enforcing quiescence on humanity: people rarely travel, rarely fight in wars, rarely do anything but live and worship him; the millennia of repression creates in humanity a deep and urgent need to explode upon the universe, scattering itself beyond the reach of any single tyrant.
The problem is how does a god die without destroying his people? If a god was to commit suicide, his worshippers would commit suicide with him. For a god to fade, so would his people. The only way Leto can die and not take his followers with him is in revolution, so he breeds for the person who will overthrow him: the daughter of his major-domo, Siona. Siona is the first human to carry a gene that makes her invisible to prescience, and thus uncontainable by it. Since she cannot be seen in a vision, she cannot be controlled by a vision. The tyranny of prescience will end with her--humanity can never again be bound by a Leto.
Siona's partner in revolution is Duncan Idaho, the constant companion of Leto throughout the years. For his entire reign, Leto has had a ghola of Duncan in charge of his Fish Speaker army. The Duncans represent the old Atreides loyalty, along with everything vital in humanity, so something in the Duncans always rebels against the holy blasphemy Leto has created; many Duncans die trying to kill Leto. When Siona finally arrives, she finds in Duncan a justification for revolt.
Meanwhile, the rest of the universe is scheming to kill Leto as well. The Bene Tleilaxu try many ham-handed schemes; the technocrats of Ix are smarter. They craft a human to seduce Leto. First they create Malky, a being of perfect evil, a devil to Leto's God. Malky is a charming Lucifer, and as ambassador to Leto's court he plumbs the depths of Leto's piety. In Dune Messiah, the face dancer Scytale reveals to a Reverend Mother that the Bene Tleilaxu created their own Kwisatz Haderachs, meaning "pure creatures", and discovered that Kwisatz Haderachs will die before becoming their opposites (and so can be killed by manipulating them into betraying themselves). Malky's purpose is to get Leto to turn on his holy creation, but he fails because the Ixians don't realize that Leto, more than anyone, knows the blasphemy he has created.
At the same time as Leto is breeding an invisible human, Ix invents another solution: no-chambers. A no-chamber is an electro-mechanical construct that hides its contents from prescient vision. Inside the first no-chamber, the Ixians grow their next attempt: Malky's niece, Hwi Noree, who shall replace him as Ix's ambassador. Hwi is the opposite of Malky, a creature of pure goodness. Where Malky failed, Hwi succeeds. Leto falls in love with her, and plans to marry her. In so doing, he weakens his godhood enough to allow Siona's revolt the possibility of success.
Leto plans to wed Hwi in what remains of an old Fremen village near the former Sietch Tabr. The desert disappeared long ago, except for a preserve in which no sandworms roam. Leto plans a journey by foot to the village where he has sent Duncan and Siona, hoping they will breed. But at his arrival, a Fish Speaker, thinking that it was a test, uses a lasgun to destroy the bridge Leto and Hwi are crossing, dropping them into a river. The water destroys Leto's sandworm body, decomposing it into the sandtrout that will lock up the water in their bodies, recreating the conditions for the sandworms to re-appear, each with a pearl of Leto's consciousness inside it. Leto dies, his last vision that of the Golden Path, shining brightly in humanity's future.
After Leto's death, driven by the famine times and the pent up desires of thousands of years of Leto's peace, humanity explodes out of the universe in waves of migration known as the Scattering, as predicted by the Golden Path.