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In the fictional universe of Warhammer 40,000, psykers are individuals with some form of psychic ability. Psykers draw their power from the Warp, a parallel universe of pure energy. Because the Warp is inhabited by daemons, psykers are often at risk of insanity or daemonic possession.
Psykers of Different Races 
Parallel to the material universe in which the mortal creatures of the galaxy live is a realm of pure psychic energy known as the Warp. Almost all sentient beings have a mental connection to this realm. Individuals that have an especially strong connection are known as "psykers", and with training they can develop supernatural powers such as telepathy, divination, or pyrokinesis. The substance of the Warp is influenced by the thoughts and emotions of all sentient creatures, and these energies sometimes coalesce into entities known as daemons and Chaos Gods. Most of these Warp entities are predatory and constantly seek to influence the mortal realm by manipulating psykers, whose strong connection to the Warp make them potential conduits to realspace. Every psyker is in danger of going insane or being possessed by a daemon if he does not have the proper training and strength of will.
Most races in the Warhammer 40,000 universe employ psykers on the battlefield; some are inevitably more potent than others. There are three exceptions to this: the Tau, the Dark Eldar, and the Necrons.
The Imperium of Man 
Ordinary humans fear and hate psykers because, without special training, they are prone to insanity and daemonic possession. Psykers are so hated that they risk being lynched if they are not taken into custody of the state. Psykers who practice their powers without the sanction of the state are termed "witches". Under Imperial law, all planets must round up their psykers and hand them over to the Inquisition, who make regular visits in their Black Ships. Those psykers who are deemed corrupt or uncontrollable are summarily destroyed; those who are too weak to be of practical use are sacrificed to the Emperor; those who display both power and control are trained become servants of the state. Imperial Guard Regiments and Space Marine Chapters use psykers in combat roles. Astropaths provide faster-than-light psychic communication. Navigators are needed to pilot starships through Warp space. Most humans do not appreciate the critical roles that sanctioned psykers play in the Imperium, because most do not come into contact with them and are not even aware of their existence (ignorance is common in the Imperium).
Pyskers who are too weak to fulfill the aforementioned roles are sacrificed to the Emperor. The Emperor feeds on their souls and uses this energy to project a psychic beacon, the Astronomican, through the Warp. Imperium starships need this beacon to travel through the Warp, which otherwise has no fixed reference points for navigation. Thousands of psykers die daily in this fashion. Because the Astronomican is critical to the Imperium's infrastructure, this is seen as a small price to pay.
Astropaths are long-range telepaths who are trained to transmit and receive messages across interstellar space. Though messages are often distorted, delayed, or lost altogether in transmission, astropathy is the only means of fast interstellar communication available to the Imperium. Astropaths can be found on nearly every Imperial ship and world. Because the transmission range of individual astropaths is relatively limited, they relay messages through an astropathic chain, or form choirs (conclaves of astropaths) to boost their range.
To become an astropath, a candidate psyker is brought before the Emperor to be soul-bound: the psyker's mind is reshaped by the Emperor, thereby enhancing her/his telepathic potential and girding the psyker against predation by Warp entities. This painful process is sometimes fatal and often destroys one or more of the psyker's senses (almost all of them are blind). Because of the psychic, mental, and physical strain involved in this method of transmission, most active astropaths are short-lived.
Navigators form an ancient lineage of mutants who possess a third eye in the middle of their foreheads which allows them to look directly upon the Warp and perceive the Astronomican. Without a Navigator to guide a ship, interstellar travel is very slow and dangerous. Navigators cannot interbreed with regular humans due to the recessiveness of the navigator gene, and thus form an endogamous caste. The Navigator Houses are very powerful entities, even boasting a representative among the High Lords of Terra. Navigators are perhaps the only class of mutant to enjoy high status in an otherwise very intolerant society.
Imperial Guard 
The Imperial Guard employs sanctioned psykers in combat roles. On the battlefield, Sanctioned Psykers can advise an officer and guide their command, protect him from psychic attack, or fire bolts of lightning from their hands. Also The Imperial Guard, with the latest version of the tabletop game have begun using Psyker Battle Squads which are a larger group of lower ranking Sanctioned Psykers (typically between 5 and 10) that pool their powers together for each attack. These psykers have less training than Space Marine Librarians and are thus more vulnerable to the perils of the Warp. They are such liabilities that they are often accompanied by commissars, who will swiftly execute a psyker who is on the verge of becoming a danger to his fellows.
Space Marines 
Psykers are employed by the Space Marines in the form of Librarians. These individuals are chosen from among the most powerful and physically fit psykers. Librarians are responsible for maintaining the records of the Chapter they belong to, and can also use their talents to divide psychic mutants into those that are dangerous and those that can be utilised by the Imperium. Librarians, having much better equipment (Aegis hood circuitry, etc.), willpower, protection and training, combined with the enhancements of a Space Marine, are much less prone to the perils of the Warp than other Imperial psykers.
On the battlefield, Librarians are fearsome opponents, able to unleash devastating psychic powers and augment their own physical prowess. They are also skilled at defending themselves and their battle brothers from psychic attack.
Attitudes towards psykers among Space Marines vary. The Black Templars despise them and do not have any Librarians among their ranks (though they use astropaths and Navigators out of necessity). The Blood Ravens, by contrast, venerate their Librarians, and their Chapter has an unusually higher number of them.
The Inquisition 
Though they are tasked with hunting down rogue psykers and collecting latent ones for training to aid the Imperium, many inquisitors are psykers themselves, though with the strength of will and psychic resilience to control their abilities. Inquisitor Gideon Ravenor is an especially potent example. Like much of their organization, one Inquisitor's psychic abilities can vary greatly from another's. Some specialise in unleashing torrents of fire while others, like Inquisitor Gregor Eisenhorn, specialise in powers that aid them in interrogations or to fool an enemy into stepping out from cover, dropping his weapon, or any number of other subtle 'mind tricks'.
Grey Knights 
The Grey Knights Chapter of Space Marines is wholly made up of psykers: being a psyker is a condition for recruitment. This makes the already rigorous process of candidate selection even more so, to the point of causing all candidates to completely lose their memories of their lives before their selection. Surprisingly, despite being entirely composed of psykers and dealing exclusively with daemonic threats, there has never been a recorded instance of a Grey Knight falling to Chaos. This is attributed to the hyper-devout faith in the Emperor of Mankind possessed by all Grey Knights that is conditioned into them within the recruitment process into the Inquisition. Grey Knights are also the only Space Marine Chapter that is included within the Inquisition and as such receive even further processing in order to prevent corruption of the Inquisition as a whole.
In the game, every Grey Knight unit or character may utilize individual powers (and some may use more than one per turn). Being a Psyker allows even a newly initiated Grey Knights to use a Nemesis Force Weapon, a powerful combat weapon with various, specialised forms. The Grey Knights also have access to many more specialised forms of wargear, usually unavailable to other forces, such as the Nemesis Dreadknight, a living behemoth of adamantium piloted by a single Grey Knight Terminator. In addition, much of their equipment and psychic potential has further, more potent effects if used on Daemons.
Witch Hunters 
The Sisters of Battle do not have any psykers among their ranks, but they sometimes use psyker auxiliaries, most often members of the Inquisition.
Every Eldar has latent psychic ability, and they can all speak telepathically and sense emotions. They possess some of the most powerful psykers in the galaxy. Although Eldar have more powerful psychic abilities than humans, they are not as vulnerable to daemonic corruption. All Eldar must commit themselves to a "Path" - a career to which they commit until they master it. The emotional discipline involved protects the Eldar's mind and body from daemonic influence. The most powerful Eldar psykers are the Farseers, whose prophetic powers guide every major decision the Eldar make.
Dark Eldar 
The Dark Eldar suffer a unique curse wherein their souls are being constantly drained by the Chaos God Slaanesh. To stem this consumption, they do not develop their natural psychic potential, as this would strengthen Slaanesh's link to them. Thus, Dark Eldar armies have no psykers among their ranks.
Legions of Chaos 
The forces of Chaos host a great many mutants among their number. Chaos psykers can perform various tasks such as summoning daemons and unleashing devastating powers upon the enemy. Most psykers amongst the forces of Chaos, especially among the Chaos Marine legions, are sorcerors. The Chaos God Tzeentch is devoted to sorcery; legions worshiping Tzeentch, such as the Thousand Sons, possess a large number of psykers who can aid in battle. Other legions, such as the World Eaters, denounce magic and psychic ability in favour of the wanton bloodshed of close combat, and the Iron Warriors often sneer at sorcery. Chaos sorcerers can unleash potent powers and are fearsome individuals, but are often mutated beyond recognition. Tzeentch Sorcerers are especially potent with the art of dark magic
All Orks are relatively powerful latent psykers and are capable of pooling their psychic ability to enhance the ferocity of their attacks. This field of psychic power is known as the Waaagh!, and grows more powerful when Orks engage in battle. However, they do have true, active psykers known as Weird Boyz. Weird Boyz do not draw their power directly from the Warp, but instead from the aforementioned Waaagh! field. The trouble with Weird Boyz is that they cannot shut out this flow of energy, so if they do not discharge the excess by belching some fire here and there, their heads can literally explode.
The Necrons are little more than soulless robots and thus cannot use psychic powers. The Necrons also are notable for utilizing technology specifically designed for destroying psykers, such as the "pariahs".
The alien race of the Tyranids is one of the more-psychically-active races in the galaxy, if not the most, with every single Tyranid telepathically connected to all others, in what is called the Hive Mind. It is suspected that this constant and massive "psychic traffic" is what causes the Shadow of the Warp, a phenomenon that makes Warp travel and astropathy almost impossible near a large Tyranid fleet. Though the lesser creatures have no psychic abilities of their own, the larger Tyranid warriors act as focal points for the Hive Mind, capable of mind-controlling all lesser Tyranids around them. Zoanthropes, creatures supposedly evolved by combining tyranid DNA with DNA from other psychically active races like the Eldar, are extremely powerful psykers to the point that, with the passing of the years, their bodies have devolved into little more than an overly bloated head attached to an atrophied, fetus-like body incapable of even moving on its own power, therefore requiring the creature to use its psychic abilities to levitate across the battlefield. Hive Tyrants are also very powerful psykers, although not as much as Zoanthropes, and act as the core of the Hive Mind within the Tyranid swarms. Larger Tyranids, such as Dominatrixes and the mysterious Norn Queens, are also psykers of immense power, though the full extent of their abilities is a matter of speculation.
The Tau have no psykers among them. In fact, they barely have any Warp presence at all. This makes them very resistant to the mutating effects of Chaos. They know little about the Immaterium beyond its existence, and often display laughable naivete about daemons and the Chaos Gods, who in turn take little interest in the crumbs that are Tau souls. Because they have no Navigators, their starships can only make shallow Warp jumps, and thus travel five times more slowly than Imperium ships. The Tau do rule a number of client species that possess psykers, but none are employed by the Tau military.
It is often speculated that the Tau's unquestioning obedience to the Ethereal caste is due to some psychic control, though this is more likely to be a form of pheromone control.
Psyker Levels 
There are different grades regarding the power of a psyker:
- Alpha Plus
This list continues all the way down the Greek alphabet.
The power the top four grades represents is immense. A high Delta level can read the minds of a good sized town simultaneously, or crush a man to death against a wall in seconds. High grade psykers are extremely powerful, and not to be taken lightly. Beta level is the highest a human can go and still be considered sane.
An Alpha Plus, however, is a being of grotesque power. They are described in the 4th edition Rulebook as being able to "turn a man inside-out with a glance", "snap a Battle Titan in half with a flick of the wrist", and "a muttered syllable can turn an army upon itself in a frenzy of bloodlust". They are capable of destroying entire worlds - sometimes unintentionally.
There is some controversy as to whether this description should be taken as an objective description of the Alpha Plus Level of psychic mastery, or if it is intended to be read as Imperial propaganda, and just being the exaggerated view of the generally anti-psychic authorities of the 40K world. In Dan Abnett's works, where the term first appeared, the Alpha Level psyker Esarhaddon (also the name of the King of Assyria between 681-669 BC) is powerful and able to control hundreds of Imperial citizens as puppets against the protagonists, but he is not described as being powerful enough to destroy a battle Titan, and is eventually subdued (Many works of fiction show that psykers seem to specialise in certain areas of psychic ability, so it is possible that Esarhaddon specialised in mind control or mental manipulation). Abnett's works (specifically the Eisenhorn trilogy) also refer to characters having a "PQ" rating. It is unknown how this relates to a psyker's power.
Nevertheless, few Alpha or Αlpha Plus psykers can be controlled safely, or even at all. Because more powerful psykers attract proportionally more interest from warp-bound daemons, many are driven insane, tainted by Chaos, or even become possessed (examples include the Apex Twins, who were responsible for murdering scores of Imperial Regiments). They are one of the biggest threats the Inquisition has to face, and are the reason that the Ordo Hereticus was formed. Most are executed, as the threat they pose is simply too great to deal with or control in any large degree.
An example of a Beta level psyker is Sergeant Agun Soric, an Imperial Guardsman from the Gaunt's Ghosts series of novels by Dan Abnett. He is able to detect blood poisoning in a fellow trooper, but his most prominent power is his ability to write messages to himself, which are contained in a brass message shell. Even after throwing the shell away, he would soon find it wriggling in his pocket, containing a message written in his own handwriting. At first the messages contained advice or warnings, but soon began to include criticism as he began to realize what he was, and the danger he posed as an untrained psyker. He is eventually discovered and taken by the Inquisition's Black Ships. His ultimate fate remained unknown for several novels until Only in Death, where he is discovered by Commissar Viktor Hark, who shoots him to spare him the misery of living any more.
As one goes down the scale, the individual's psychic presence decreases steadily until his mind has no presence in the Warp whatsoever, and is considered soulless. Some "blanks" in fact have the opposite effect: their brains actively push the Warp away, disrupting psychic phenomena within their vicinity. Their aura of anti-psychic energy provokes instinctive revulsion in individuals with souls, especially psykers. These creatures are very rare, but some institutions such as the Inquisition and the Officio Assassinorium make use of them to counter psykers.
In Ian Watson's novel Space Marine, an individual's psychic profile is represented by a decimal value, presumably from 0 to 1, instead of using the Greek alphabet system.
Anti-Psyker Weaponry 
Several types of anti-psyker weaponry have been developed by the Imperium, Necrons and other races to counter psykers. For the Imperium, these include the Culexus Assassin and their psykout weapons, which have devastating effects on psykers. Weapons include psycannon, warp swords, and other psykers. Necrons have developed several Pylons that, once complete will completely separate the warp from the material universe, thus rendering psykers useless. On a smaller scale there are the Pariahs, Necrons made from humans with the pariah gene - this makes them, like the Culexus assassins, soulless, and severely debilitating to nearby psykers.
The Greek Alphabet system of measuring psyker potential also applies to measuring anti-psykers. The lowest recorded example so far is an Omega Minus (the polar opposite of an Alpha Plus).
Untouchables are similar to Pariahs, but are widely assumed to have less of a blunting effect (although an accurate comparison has never been made). Untouchables cancel out psychic power cast near them or around them. Some even have the ability to consciously focus their power. It is also possible for an Untouchables power to be overwhelmed by a sufficiently strong psychic force. Alizabeth Bequin, an associate of Inquisitor Eisenhorn, was psychically overwhelmed by the warp sentience of an ancient Titan she was attempting to disable. Wystan Frauka, an untouchable in the service of Inquisitor Ravenor, was also made "touchable" due to unusual psychic activity.
Lastly, the Dark Eldar, while not having any known psykers among their ranks, have developed weapons designed for psyker destruction. One of the most gruesome (and rare) is the Crucible of Malediction, a nightmarish device which contains the essence of a slain psyker tortured into insanity. When the Dark Eldar use this weapon, the psykers tormented soul is released and its power has the potential to consume any psyker on the battlefield.
Controlling Psykers 
In the Imperium of Man, psykers are typically viewed as a necessary evil by those in power. The most prominent example of this attitude can be found in the Navigator Houses, which are vital to the Imperium's survival. However, this attitude does not extend to untrained psykers within the general population, who regularly face violent witch hunts, sponsored by the Inquisition. This control is also enforced by the Ordo Hereticus, a subset of the Inquisition dedicated to control of heresy, and takes the form of regular visits to all planets within the Imperium by the Black Ships, large specially modified interstellar space ships designed to capture and transport all psykers on a planet. Within the Inquisition, withholding psykers from the Black Ships is considered a serious offence.
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