The Spacing Guild is a fictional organization in Frank Herbert's Dune universe created in a series of science fiction novels starting in Dune and ending with Chapterhouse Dune. This Dune universe has been further developed by the writing of six prequels by Brian Herbert and Kevin J. Anderson.
According to the Anderson/Herbert prequels, the Spacing Guild was created during the Butlerian Jihad by Norma Cenva, a mathematical genius with great psychic power who allowed herself to mutate using the spice melange. Though in Dune: House Corrino it is stated that her lover Aurelius Venport was the founder of the Spacing Guild, it is revealed in Dune: The Battle of Corrin that she credited him with its foundation. Cenva is depicted as so engulfed in her work that she pays no attention to the political and social ramifications of her new technology. The prequels hint that the Guild's eventual power-hunger was partly shaped by Cenva's son, who was a great (yet ruthless) businessman who took over after she mutated.
In 'Appendix A' of Dune, Frank Herbert wrote that the Guild, along with the Bene Gesserit order, was responsible for the standardization of religion in the Dune universe; they promoted the adoption of the Orange Catholic Bible and offered protection to the dissenting theologians who created this book. Nonetheless, in the same appendix, Herbert held that the Guild members themselves were atheists, and only promoted this move to promote a stable societal order from which they could profit.
The Spacing Guild has a monopoly on imperial banking and interstellar travel: with the use of melange, Guild Navigators are the only beings capable of piloting the massive Guild Heighliners safely through space. The heightened awareness and prescience the spice grants allows the Navigator to plot a safe course between the stars. Contrary to popular belief, the navigators do not themselves 'fold' space, allowing a nearly instantaneous trip. The space-folding is accomplished by Holtzman drive units activated from the navigator's chamber.
The guild is apolitical (with exceptions, noted below), since their monopoly allows them to dictate terms to all parties that preserves the economy that supports them. As the only party able to transport goods in an interstellar economy, the Guild's highest concern is that commerce continue; for commerce to continue, the Guild must continue; for the Guild to continue, melange must be available. Ultimately, the Guild's only concern is that melange continue to be mined on Arrakis. This concern is embodied in the recurring line "The spice must flow!".
Thus, the Guild holds de facto veto power over all wars and political maneuvering. Military action is permitted, as long as the Guild is paid high rates to transport the troops, but major upsets in the political order of the universe must be approved by the Guild. In Dune, Paul Atreides defeats the Emperor Shaddam IV in a battle on Arrakis, and demands first the emperor's daughter for a wife, which would make Paul heir to the throne, and second that the Emperor immediately step down in favour of his heir. The demands are coupled with a threat to destroy the spice. Since that would end all interstellar transit, the Guild sides with Paul, threatening to strand the Emperor and his troops on Arrakis if he doesn't accede.
In the original novels by Frank Herbert, the Navigators are humans who have adapted to life in zero-gravity. They have slim builds, with large webbed hands and prehensile feet. They must spend their time in an artificial zero-gravity chamber when visiting a planetary surface, as exposure to full earth gravity would be (at best) highly uncomfortable, and potentially lethal. Whether this adaptation is the result of artificial engineering or many millennia of selective breeding is not stated in the books. The David Lynch film of Dune introduced the idea of the Navigators being "mutated" by exposure to the spice, and shows three "stages" of this mutation. The final, most advanced stage is a huge, sluglike creature. Herbert is known to have liked this design, and allusions in the final Dune books may indicate that he adopted the concept for his own works.
Navigators are made prescient by the spice (a requirement of being a pilot), and are sometimes utilized as such: In Dune Messiah, a Navigator named Edric takes part in a plot to assassinate the Emperor, Paul Atreides. The presence of a prescient hides the activities of that person, and those around him, from other prescients; Edric's involvement is solely to protect the conspirators from Paul's prescient sight.
The Guild monopoly on space travel and transport and upon interstellar banking is taken as the beginning point of the Imperial Calendar.
In the fifth and sixth novels of the series, Heretics of Dune and Chapterhouse Dune, ~5000 years after the reign of Paul Atreides (a period that includes 3500 years of Leto II's reign and ~1500 years following his death), the technocrats of Ix develop technology that the Ixians and the Administrative faction of the Spacing Guild refer to as 'compilers'. These compilers perform calculations very similar to computers and, because of this similarity, almost violate the prohibitions against 'thinking machines' that were imposed following the Butlerian Jihad several millennia ago. These compilers eliminate the need for the Guild Navigators (who guide Heighliners through foldspace using melange-induced prescience) and the strategic disadvantage that this aspect of melange dependency has become because the Navigators abilities are slowly being compromised by the severe reductions in the availability of spice resulting from the destruction of Dune, the sandworms on that planet, and the strict control by the Bene Gesserit who maintain a monopoly over the largest stockpiles of melange.
The prescient rule of Leto II that lasted 3,500 years has shown the universe the perils of prescience, namely that the entire universe can be locked into the vision of a single entity, giving that entity absolute power. The Guild, facing obsolescence and suspicion, couples itself with Ix in decline; Guild navigators continue to exist, but their importance in the universe is severely diminished.
Recalling comments made by Paul Atreides in Dune, it was the Spacing Guild's obsession with the 'safe path' that led them 'ever into stagnation,' and brought on their eventual obsolescence.
In the final mission of the single-player campaign, you must fight an alliance of the Guild and the Tleilaxu; the main objective is to destroy the 'Emperor Worm', a human-sandworm hybrid bred by the Guild and the Tleilaxu.