Leto Atreides II, also known as the God-Emperor, was the son of Paul Atreides and his Fremen concubine Chani, daughter of Liet, and the twin brother of Ghanima Atreides. After the death of his aunt, the regent Alia Atreides, Leto II proclaimed himself Emperor of the Atreides Empire, and ruled for approximately 3,500 standard years, guiding humankind with his iron fist towards survival. He was named after the Duke Leto Atreides, his paternal grandfather.
Leto II and his twin sister Ghanima were born on planet Arrakis during the reign of Emperor Paul Atreides. Their mother Chani died shortly after giving birth to the two children, due in large part to complications arising from large amounts of contraceptives that were surrepticiously given to her by Paul's legal wife Irulan Corrino. Paul had anticipated Ghanima due to his prescient visions. However, Leto's arrival came as a surprise to the Emperor.
Following Chani's death, and the apparent death of their father in the desert of Arrakis, Leto and his sister became the legal responsibility of Paul's younger sister, the regent Alia. Though they were granted a fine upbringing, little attention was bestowed upon them by their aunt, who was possessed by Abomination and enthralled by her own power and the mysticism developed around the Atreides name by Paul.
Evolution on Arrakis
When they were nine years old, Leto and Ghanima gained the attention of their paternal grandmother, the Lady Jessica, who returned to Arrakis from planet Caladan to inspect them as part of her obligations to the Bene Gesserit. While visiting, Jessica sensed the latent power present in the twins, and upon the realization that they could be a threat to the psychologically unstable Alia, pressed her Fremen allies to help protect them.
At this same time, Leto's prescient powers began to emerge. Through visions and intuition he discovered the Golden Path that his father had earlier uncovered and begun to follow. However, unlike Paul, Leto developed a greater understanding of the Golden Path's implications. He sensed not only its dangers but also the painful steps that had to be taken to avoid it unraveling, which would be to the peril of humankind.
After Leto and Ghanima escaped a failed assassination attempt by House Corrino, the two split up, with Ghanima conditioned to believe her brother was dead, so that if she were to be interrogated on his whereabouts, she would not be able to tell the truth.
Leto then slipped into almost total anonymity, using the time and relative seclusion to build a foundation of power and knowledge through which the full impact of the Golden Path could be realized. To enact such steps required a strong (almost brutal) grip on power, and a longevity that would override the shortsightedness and impatience of Man.
Thus, after spending time amongst a variety of fringe Fremen elements, including The Preacher, Leto accepted sand trout upon his body and began the conversion into a human-sandworm hybrid. This transformation gave Leto immense reflexes and speed, and he was able to bound across large distances on foot.
Ascendancy to Emperor
After his transformation had progressed to a sufficient stage, Leto emerged from the desert and returned to the city of Arrakeen to confront the possessed Alia and claim the throne of the empire. After Alia managed to briefly overcome her possession and take her own life, Leto claimed the title of Emperor and promptly married his sister, to consolidate his hegemony. The marriage was purely legal, however, and Ghanima accepted Farad'n Corrino, who had been taught Bene Gesserit ways by the Lady Jessica, as her exclusive concubine.
Reign of Leto II
Through the evolution of Leto's body and powers (both prescience and access to possibly every dead ancestor in history), he was rendered infertile and slowly evolved into what appeared to be a sandworm with a human face and appendages of limited strength and range. In return, his lifespan, speed and reflexes were significantly increased, and during a reign of some 3,500 years Leto II successfully guided humanity through a period of vulnerability and potential danger.
During his rule, Leto II outlawed the order of Mentats and began shutting down any renegade schools whereever found. The art of being a Mentat had not yet been eliminated, however, surviving through underground schools. Moreover, Leto took control of the Bene Gesserit breeding program, outlawed interstellar travel without his permission, and forced populations to be planet-bound through garrisons of Fish Speakers on all planets of the Imperium.
Toward the end of his reign Leto sensed the need for the next step on the Golden Path, and knew that his rule must end for further progress to occur. Thus using Siona - a distant descendant of Ghanima and her concubine Farad'n - and a Duncan Idaho ghola created by the scheming Tleilaxu, he sowed the seed of the Atreides genes for future generations. Additionally, he began the reversal of the terraforming on Arrakis, and witnessed the planet begin to revert back into a desert world from the realised Fremen dream of a lush paradise.
Ultimately Leto lost his own life when Siona ordered the bridge he was crossing to be cut by a lasgun and he fell into the Idaho River. Following Leto's death, a chain of dramatic events took place over a span of 1500 years, that included the Famine Times, the Scattering, and the recreation of the sandworms on Arrakis, which emerged from the sandtrout that escaped from Leto's body when he fell into the water.
During and after his reign, Leto II was viewed as an extremely controversial figure. Many of the established power brokers who existed at the start of his rule were either destroyed or significantly weakened, due to his draconian tactics and a monopoly on the Spice Melange. To his admirers, Leto II was known as the God-Emperor, but to his enemies (including the Bene Gesserit) he was labeled the Tyrant of the known universe.
It was also said that within each sandworm that grew on Arrakis after his death, a pearl of his consciousness existed. This theory was supported when, approximately 1500 years after his death, an Arrakeen girl named Sheeana, who was a direct descendant of Siona and Duncan Idaho, had the power to control the worms.
The religion of the God-Emperor continued on Dune, until the attack on the planet by the Honored Matres, and also with those returned to the Old Empire from the Scattering, who called Leto II Dur or Guldur.