Navigator

Navigators (Homo navigo) are Warp tainted freaks 3-eyed psykers that are responsible for keeping your sorry-ass ship from getting lost in the warp and stop the evil little critters from attacking. Well-known as arrogant cock-suckers who have families richer than Bill Gates and twice as corrupt as any 3rd world country's government.

Also, one of those parts of the Imperium of Man blatantly stolen from Dune.

In the beginning

During the Dark Age of Technology, humankind experimented with human genes to allow a mutant race of psykers to come into existence. The most outward sign of this mutation was the third "Warp-eye" which could navigate the currents of the Warp and get people from one planet to another without getting annihilated by daemons. These people would be able to pass their psychic gifts to their children, but only if they procreated with another Navigator. The various families of Navigators thus formed the Navis Nobilite, wealthy families who were essential to star flight. However, as Navigators get older and more exposed to the warp, they start showing less...pleasant mutations (scales, tentacles, the like).

The Imperium

When the Emperor set about uniting Terra and preparing to go to the stars, he was presented with a bit of a problem. Part of the Imperial Truth he was sprouting maintained that humans were destined to rule the stars, and he was still centuries away from getting the Webway project up and running. Simply put, the Emperor needed the Navigators, even if he was fully aware of how "age before beauty" was a horrible lie to the Navigators. Deciding "fuck it, we need space travel", the Big E went to the Paternova (the leader of the Navis Nobilite) and made an agreement: "Keep to yourselves in your part of Terra and your mutant Elders out of sight, and we'll pay you to fly our starships and protect you from some of the more zealous anti-mutant groups". The Paternova agreed, and thus humans can fly through the Warp, using the Astronomican to navigate.

In the 41st millennium, the Navigators are part of the High Lords of Terra, represented by the Paternoval Envoy. A lot of people in the Administratum, the Ecclesiarchy, and the Inquisition aren't too thrilled with this, but they can't really do anything to the Navigators without irreparably crippling the Imperium. The more extreme ones still try to do something about it. Imperial dogma is frequently contradictory. Pointing this out is heresy.