In the Dune universe created by Frank Herbert, Guild Navigators or Steersmen are humans, mutated through the consumption of massive amounts of the spice melange, who are able to safely navigate interstellar space in a heighliner using prescience.
Guild Navigators are continuously immersed in highly-concentrated amounts of spice gas, which causes their bodies to atrophy and mutate over time, their heads and extremities elongating. The first external sign of melange-induced metabolic change is visible in the eyes, as the drug tints the sclera and iris to a dark shade of blue, called "blue-in-blue" or "the Eyes of Ibad, appearing almost black.
In Dune chronology, these effects are first experienced by Norma Cenva at the end of The Battle of Corrin:
Her direct physical senses were deadened, and Norma no longer cared about taste, touch, or smell ... She found it remarkable to see webbing between her fingers and toes. Her face, once blunt-featured and later flawlessly beautiful, now had a small mouth and tiny eyes surrounded by smooth folds. Her head was immense, while the rest of her body atrophied to a useless appendage.
Because the Spacing Guild uses mainly envoys and ambassadors in their dealings (presumably to hide the nature of the Navigators), Guild Navigators were historically seldom seen. Although their appearance is a mystery in the original novel Dune, a Navigator is fully revealed in the first chapter of Dune Messiah. Here, the Guild Navigator Edric is described in his tank of spice gas as "an elongated figure, vaguely humanoid with finned feet and hugely fanned membranous hands — a fish in a strange sea." In Chapterhouse Dune references are made to a Navigator meeting-hall on the planet Junction; it is stated that the hall is vast, in order to accommodate the very large tanks in which Navigators are obliged to live while on a planet's surface. Due to the genetic change a navigator undergoes after being subjected to concentrated spice gas, they are no longer able to process oxygen. They can only survive in a spice gas environment; they would literally choke to death in open air. The size of the tanks is partly to prevent a sense of claustrophobia (elsewhere in the Dune saga it is stated that Navigators spend most of their lives in space, presumably in the weightless environment of a Heighliner's control room) and partly because of their physical size. This ties in with David Lynch's depiction of a Navigator in his tank in the opening section of his film version of Dune. In the film, the Navigator's mutation affects his entire body, and he resembles a large newt or grasshopper with a heavily deformed head and V-shaped mouth. His limbs are vestigial. The Navigator puppet in this film did not appear to have the all-blue eyes of a spice addict.
It should be noted that in an unused passage from Dune Messiah published in The Road to Dune, Edric is described as surviving in air, without spice gas, once his tank is destroyed. His prescient abilities are practically useless in this state.
In the Prelude to Dune novels, D'murr and C'tair Pilru (twin sons of Ambassador Cammar Pilru of Ix) take the initial examination to become Navigators, but only D'murr passes. From the moment he enters training, D'murr is told he will never communicate with his family again; he eventually becomes a full Navigator.
In House Corrino, D'murr is piloting one of two heighliners which Hasimir Fenring uses to secretly test the synthetic melange created by the Tleilaxu in their Project Amal. The flawed spice disrupts and confuses D'murr's thoughts, feelings and prescience. He is compelled to send a telepathic message to his twin C'tair (who is hiding on Tleilaxu-conquered Ix) that his own heighliner carries rightful Ixian ruler Rhombur Vernius, on his way to free the planet. Disastrously, the first heighliner emerges from foldspace at the wrong point, striking the defensive shields of Wallach IX and plummeting into the atmosphere to its destruction. Affected by the tainted melange, D'murr misguides his ship out of the known universe and collapses. As his spice supply is replaced with real melange, D'murr senses with alarm that 'the enemy has seen us' (this is presumably the first reference to the Unknown Enemy that, millennia later, chases the Honored Matres back to the Old Empire in Heretics of Dune and threatens to destroy humanity). D'murr uses the last of his strength to return the ship safely to Junction, home of the Guild Headquarters, and dies.
In Hunters of Dune the 2006 Herbert/Anderson sequel to Chapterhouse Dune, the Navigator Edrik fears his kind's obsolescence when the Spacing Guild itself (pressured by a shortage of melange) begins funding the development of superior Ixian navigation technology that would not require Navigators. Seeking an alternative source of spice to break the Bene Gesserit monopoly, he meets with Uxtal, the last of the Lost Tleilaxu, hoping that he can rediscover the method of producing melange in axlotl tanks (a secret believed lost when the Bene Tleilax were destroyed by the Honored Matres). However, Uxtal is in the forced service of the Matre Superior Hellica; her price for his expertise is Edrik's help transporting a certain cargo. He agrees, delivering by heighliner the Obliterators that destroy the planet Richese, where the Bene Gesserit are mass-producing weapons and armed battleships. Uxtal is ultimately unsuccessful, but the ghola he creates of deceased Tleilaxu Master Waff later offers Edrik something better in exchange for sanctuary — the genetic knowledge for the Guild to create their own, optimized sandworms to produce melange.
In Sandworms of Dune (2007), the sequel to Hunters and finale of the original Dune series, the Spacing Guild has begun replacing its Navigators with the more cost-effective Ixian navigation devices and cutting off the Navigators' supply of melange. More and more Navigators are dying from withdrawal of the spice — including Ardrae, "one of the oldest remaining Navigators" — and many defect and disappear into space rather than allow the devices on their ships. All are unaware that Face Dancer infiltrators are behind the plan, plotting their own takeover of the universe.
Waff works in secret, hidden on Edrik's own heighliner, on genetically engineering his "advanced" sandworms. He accomplishes this by altering the DNA of the sandtrout stage and creating an aquatic form of the worms, which are then released into the oceans of Buzzell. Adapting to their new environment, these "seaworms" quickly flourish, eventually producing a highly-concentrated form of spice, dubbed "ultraspice."
Edrik and the ultraspice are later intercepted by Face Dancer leader Khrone, who seizes the valuable optimized melange. He incapacitates Edrik by damaging his tank and releasing its spice gas, soon destroying the entire heighliner to rid himself of the Navigator altogether.