Psyker

Psykers are individuals with psychic abilities in the Warhammer 40,000 fictional universe. Psykers draw their powers from the Warp, and hence are often in danger of daemonic possession and insanity.

Psykers of Different Races
Most races in the Warhammer 40,000 universe employ some form of psykers to their use on the battlefield; some are inevitably more potent than others. There are two exceptions to this: the Tau and the Necrons. The Tau have yet to bear any psykers and Necrons have zero contact with the Warp as a whole.

The Imperium of Man
The Imperium employs various 'psykers' throughout the Imperial Guard Regiments and Space Marine Chapters to effective use on the field of battle as well as for civil duties. Navigators are required to cross safely through the Immaterium. Astropaths provide faster than light communication. Thousands of souls from psykers are required to maintain the Astronomican, which broadcasts an enormous signal through the warp which is used as a point of reference by ship navigators. However, the Imperium of Man also executes countless psykers as they are seen as mutants, potential threats, or gateways to Chaos.

Astropath Origins and Role
Astropaths are chosen from the multitudes of psykers brought to Holy Terra on the black ships due to their great power. Once chosen, Astropaths are spiritually joined to the Emperor. This is why only powerful psykers can become astropaths, as such an experience can be fatal. Many do die, while most survivors lose their eyesight.

Being joined with the Emperor is necessary however, as it greatly heightens Astropaths' already formidable powers and gives them the ability for which they were chosen - to send psychic messages through space across large distances, thereby becoming the main network of long distance messaging in the Imperium. Most ships have an Astropath on board and most planetary governments have access to Astropaths.

Without the Astropaths' unique ability, long-distance communication in the Imperium would be impossible. However their powers are still susceptible to the local conditions of the Warp (such as warp storms) and as such, their messages can be greatly delayed or lost. Due to the strenuous nature of their job many Astropaths are physically frail and can die quite young through exhaustion.

Imperial Guard
Although psychic mutation is frowned upon within the Imperium, certain mutants are recruited into use by the Imperial Guard as Sanctioned Psykers. These mutants may draw upon the power of the Warp, unleashing it upon their enemies. They are also called upon to advise high-ranking officers, with varying degrees of success. Their limited training and equipment leads to them being more easily preyed upon by the perils of the Warp than perhaps the much more powerful Librarians of the Space Marine Chapters. Each time they utilise their power, they put themselves at considerable risk and are prone to have their minds destroyed in the process. Some Psykers in rare cases have even literally exploded from overloading their bodies with warp energy, taking out entire squads nearby. That aside, they are still a potent force when deployed and used properly. 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.

Outside of the tabletop game, Imperial Guard Sanctioned Psykers appear in the video game Dawn of War as members of the Imperial Guard faction.

Space Marines
Psykers are employed by the Space Marines in the form of Librarians. These individuals are chosen from among the most powerful 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.

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, Inquisitor Gideon Ravenor being 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
Perhaps the faction that has more psykers than any other imperial faction, the Grey Knights Chapter is wholly made up of psykers: being a psyker is a condition for recruitment. Due to the entire chapter being psykers the rigrous selection of becoming a Space Marines is increased all that much, with all selected battle-brothers having no memory of their past and many falling before gene-seed implantation can be complete.

Eldar
Among all the races, the Eldar possess some of the most powerful psykers, using their ability to divine the future to shape their fate and also unleashing devastating powers on those that oppose them, however the sheer power they generate combined ensures that they must be cautious unless they attract the attention of Chaos and so are forced to become highly diciplined and regulated in psychic usage. Eldar Psykers come in the form of either Farseers or Warlocks, the former being the more powerful. Farseers will lead the Eldar warhost against the enemy and shape the course of battle with their powers. Warlocks are typically bodyguards and possess lesser, normally defensive powers. The Ulthwé Craftworld is said to possess the most and the best Eldar Psykers, due to its proximity to the Eye of Terror, and they can divine much further into the future and with better clarity than that of other craftworlds. The Craftworld Iyanden has a variant of Warlocks, called Spiritseers. They differ very little from their normal counterparts. Spiritseers can guide Wraithguards in combat, better directing their awesome firepower for optimum efficiency.

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. The Chaos God Tzeentch is devoted to sorcery; legions worshiping Tzeentch such as the Thousand Sons possess a large number of psykers that 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.

Orks
Known as Wyrd Boyz, Ork psykers differ from those of the other races in that they don't draw their power from the Warp, but instead from the latent psychic powers that all Orks have. The Wyrd Boyz then channel this Waaagh! energy to inflict damage upon their foes, but often end up killing themselves in the process.

Necrons
The general belief is that Necrons do not have Psykers, as they are little more than robots, and are described as having virtually no souls; meanwhile, C'tan are described as considering warp energies to be "anathema". The Necrons also are notable for utilizing technology specifically designed for destroying psykers, specifically the "pariahs". However, some contradictory evidence comes from the novel Caves of Ice, in which the Necrons emit a fear inducing telepathic ability, from which the unlikely hero of the novel, Ciaphas Cain is saved only because of the presence of a "psychic blank" in the vicinity.

Tyranids
The alien race of the Tyranids is one of the most 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 supossedly 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 a humongously bloated head attached to an atrophied, fetus-like body incapable of even moving on its own power, therefore requiring for 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.

Tau
Tau have no known psykers. Indeed, in the video game Warhammer 40,000: Fire Warrior, it is revealed that the Tau are naturally resistant to the mutating effects of Chaos, having almost no psychic presence in the warp at all. However, this does not prevent the Tau Ethereal, Ko'Vash, being blasted apart by a burst of Chaos energy. It is rumoured that the Ethereals are capable of utilising warp power, as it is suggested that this is how they are able to cause such loyalty in all of the Tau, although it was suggested in the Warhammer 40,000 book, Xenology, that the Ethereals have a large scent-based organ in their head that allows them to control other Tau through pheromones.

Psyker Levels
There are different grades regarding the power of a psyker:
 * Apex
 * Alpha Plus
 * Alpha
 * Beta
 * Gamma
 * Delta
 * Epsilon

...and so on 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.

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 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 Esharddon 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.

However, there are very few A or Α+ Psykers that 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, a Vervunhiver in the series Gaunt's Ghosts 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. Sometimes the message contained advice or warnings, and sometimes it was a self-critisim. His fate is unknown, though at the end of the book Sabbat Martyr he is presumably executed.

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. 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.

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.