Psyker

Psyker is a catch-all term used to describe any sentient being in the Warhammer 40,000 universe in possession of some degree of psychic ability. Psychic powers are sometimes referred to as sorcery, particularly when they are gained or enhanced by the entities which exist in the Warp. By extension, psykers are sometimes referred to as "witches" by the Imperium of Man and the Aeldari, particularly when such powers are used either with little regard for the dangers they might pose to others or in the pursuit of the agenda of the Ruinous Powers.

All psykers draw their power from the Immaterium or the Warp, the so-called "realm of souls" that exists alongside realspace. As such psykers, particularly human psykers, are often in danger of possession by the daemonic entities of the Warp, insanity or even corruption by the Dark Gods.

Psyker Grades
There are different grades used to incrementally rank the power of a psyker within the Imperium of Man, ranked from the most powerful to least powerful, top to bottom: The grades continue on down through the letters of the Greek alphabet. Grade Sigma and below are levels of "anti-psychic power". These people are more commonly referred to as "psychic blanks" since they cannot be detected, manipulated or affected by psychic means and possess the Pariah Gene. The higher-grade psykers hold immense power. A high Delta can read the minds of everyone in a small town simultaneously, or can crush a man to death against a wall within seconds, and such powers only increase from there. Beta-grade psykers represent the peak psychic ability a regular human can attain and still retain their sanity.
 * Alpha Plus
 * Alpha
 * Beta
 * Delta
 * Epsilon

An Alpha Plus-grade psyker, however, is a being of unimaginable power. They have been described 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 blood lust." If the stories are to be believed, an Alpha Plus-grade psyker is capable of destroying entire worlds - sometimes unintentionally. However, there is some controversy as to whether this depiction should be taken as an objective description of the Alpha level of psychic mastery, or if it is Imperial propaganda, and just the exaggerated view of the generally anti-psychic authorities of the Imperium.

There are very few Alpha or Alpha Plus psykers that can be controlled safely, or even at all. This is because the more powerful the psyker, the more likely they are to attract interest from Warp-bound daemons, leading to insanity, Chaos taint or, in worst cases, possession. They are one of the biggest threats the Inquisition has to face. Most psykers are executed, as the threat they pose is too great to deal with or control to any large degree. The Emperor of Mankind would be considered an Alpha Plus psyker, though His power is far beyond what would be considered normal for an Alpha Plus psyker.

Psykers of the Galaxy
Most intelligent races in the Warhammer 40,000 galaxy employ psykers to some extent. The exceptions to this rule are the Tau, the Necrons and the Drukhari. The Tau have yet to develop psykers of their own, though rumour has it the Etherial caste possess psychic abilities. These rumours were proven false when Adeptus Mechanicus researchers discovered a large, diamond-shaped, scent-emitting organ in their head which, it is believed, excretes pheromones that are used to control the rest of the Tau species.

The Drukhari, also known as Dark Eldar, do have psychic powers of their own. However, due to their lack of protection from Slaanesh, many fear to use such powers, believing that it will hasten their own demise by feeding Slaanesh. This is reinforced as one of the few laws in Dark Eldar society. Any Drukhari who manifests even the slightest amount of psychic ability can expect to find themselves in the gladiatorial arenas of the Wych Cults or the Haemonculus laboratories.

The Necrontyr may have possessed psykers in their life. However, since the entire race turned themselves into machines on the order of the C'tan, such abilities no longer appear among the necrons. This is due to the lack of a soul - the main link between realspace and the Immaterium. Despite the lack of psykers, the necrons have a powerful anti-psyker weapon in their elite guard, the Pariahs. These horrific monstrosities are a merge between captured humans who possess the Pariah Gene and the living metal Necrodermis that makes up all necrons.

Imperium of Man

 * Gregor_Eisenhorn.jpgpaths - Short for Astro-Telepath, astropaths serve throughout the Imperium, making up a psychic communication network stretching from Holy Terra to the most distant planets loyal to the Emperor. New astropaths are chosen from the multitude of psykers born across the Imperium yearly, and are brought to Terra on the Black Ships for training. This training comes from the Scholastia Psykana, the teaching division of the Adeptus Astra Telepathica, and is followed by what is known as "soul-binding." This is a psychic ritual that transfers some of the Emperor's vast power and mental control to the astropath. Such an experience is physically traumatic for the psyker, and not all survive or maintain their sanity. Even among the survivors, the soul-binding leaves a mark, usually in the form of burning out the optical nerves, leaving almost all astropaths physically blind. Their increased psychic senses tend to make up for this blindness.
 * Sanctioned Psykers - Some psykers recruited into the Adeptus Astra Telepathica are instead trained to work alongside the Astra Militarum as sanctioned psykers. These psykers may draw upon the power of the Warp, using it as a weapon against the enemies of the Imperium. They are also called upon to advise high-ranking Imperial Guard officers at times. Their limited training and equipment makes them more vulnerable to the perils of the Warp than the much more powerful psyker Librarians of the Space Marines. Each time a Sanctioned Psyker uses their power, they put themselves at considerable risk of destroying their own minds in the process. In rare cases, some Psykers have even literally exploded from overloading their bodies with Warp energy, taking out entire Guard squads with them. 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. Despite their use, Sanctioned Psykers are at best tolerated, and at worst loathed, by their companions. They are never allowed to stray far from the regiment's commissar, who can and will execute the psyker at the slightest sign of Warp taint, allegedly for the psyker's own good.
 * Primaris Psykers - If a sanctioned psyker manages to survive a term of service in the Imperial Guard, he will be returned to the care of the Adeptus Astra Telepathica, who will then test the veteran psyker thoroughly for mental fortitude and psychic might. If the psyker survives these tests, he will be trained to become a Primaris Psyker, and taught how to harness the power of lesser psykers and make it subservient to his own. When he rejoins the Imperial Guard, the primaris psyker will be able to form a "circle" or "coven" with other sanctioned psykers. With the power of the circle at his disposal, the primaris psyker's abilities are increased to levels beyond what a psyker of their grade should be capable of. Despite this increase in their power and worth, a primaris psyker is still under the suspicion of the regiment's commissar, and may still find himself executed by a bolt pistol shot at a later date.
 * Librarians - Librarians are the psykers employed by the Adeptus Astartes chapters. These individuals are chosen from among the most powerful psykers in the Imperium, and are responsible for maintaining the records of their chapter, hence their name. Due to their equipment, willpower, protection and training, as well as the genetic enhancements of a space marine, librarians are much less prone to falling prey to the perils of the Warp than other Imperial psykers. On the battlefield, librarians are fearsome opponents, able to unleash devastating psychic powers, psychically augment their own physical abilities, or even erect psychic barriers to protect themselves or their Battle-Brothers from physical or psychic attacks.
 * Grey_Knight.jpgsitors - Whilst not a requirement of their station, many inquisitors are psykers themselves. The abilities of Inquisitorial psykers vary. Some have abilities more in line with sanctioned psykers or Librarians, able to destroy their enemies with lightning conjured from their fingertips. Others, meanwhile, have abilities that support their role as inquisitors, in the form of subtle "mind tricks" that can cause their foes to drop their weapon or give up valuable information they'd rather the inquisitor not know.
 * Grey Knights - The Grey Knights are a space marine chapter composed entirely of psykers, that serves as the chamber militant for the Inquisition's Ordo Malleus, the Daemonhunters. Due to this added requirement, the selection and training usually undertaken by space marine neophytes is even more arduous, and few survive to become full Grey Knights. However, this rigorous selection process also has some merit, in that, despite the Grey Knights being more powerful than the Chief Librarians of most chapters, not a single one of them has fallen to Chaos in the millenia the chapter has existed.

Aeldari

 * Macha.pnger - Those that walk the Path of the Seer in Eldar culture are referred to as Farseers. Masters of prediction, the farseers are the leadership of most craftworlds of the Aeldari; only Biel'tan, the most militaristic of the craftworlds, truly breaks from this tradition. Through the use of complex wraithbone runes, the farseers can perform their divinations even in the heat of battle, guiding their people to victory wherever possible.
 * Warlock - It is not unknown for Asuryani walking one of the paths to turn towards another path. When an aspect warrior turns to the Path of the Seer, many find it hard to follow the path to it's expected conclusion. These warrior-seers instead follow the Witch Path to its most violent conclusion, becoming Warlocks. Most warlocks serve as a farseer's guardian, protecting the farseer with a mix of defensively-oriented psychic abilities and their own skill with blade and gun.
 * Harlequin_Shadowseer.jpgtseer - Among those who follow the Path of the Seer, some are called to commune with the dead, serving as Spiritseers - a role that requires them to command and commune with the craftworld's many wraith constructs, and the souls trapped in spirit stones or the craftworld's Infinity Circuit. According to Eldar mythology, the first Spiritseer was a seer from the craftworld Iyanden. After the death of his lover, he plunged his mind into the Infinity Circuit to be with her. There, he discovered his true calling - to bridge the gap between living and dead Eldar. On the field of battle, spiritseers command the wraithguard, using the abilities to protect the greater Aeldari warhost in the process.
 * Shadowseer - The equivalent of farseers amongst the Harlequins, the Shadowseers are masters of manipulation, turning their opponents against each other, blinding them and other such abilities. Unlike their Asuryani opposite, shadowseers aren't concerned with guiding the Aeldari to victory, but rather, to ensuring all around them, whether Aeldari or not, follow the mythic roles they unconsciously follow. An example of this can be found in the novel Primogenitor, from the Fabius Bile trilogy, where the shadowseer Sylandri Veilwalker assists Fabius Bile, a man who is technically considered a follower of Chaos despite his refusal to consider the Ruinous Powers as gods, into attacking Craftworld Lugganath, thus thrusting him into the role of the King of Feathers, with the final goal being for Fabius to take leadership of the Emperor's Children legion, though for reasons that remain unknown.

Chaos

 * Sorceror_(1KS).jpgic Psykers - Not all psykers in the Imperium are brought onto the Black Ships. Those who escape capture are almost guaranteed to fall to Chaos. These psykers are generally found amongst renegade militias, fighting alongside traitor guardsmen, chaos mutants and worse. Their powers are generally on par with those of a primaris psyker, though they are much more at threat of possession due to actively communing with the Dark Gods.
 * Sorcerors - Chaos Sorcerors are the equivalent of Space Marine Librarians within the forces of the Heretic Astartes. Blessed by the Dark Gods, these warriors are masters of death, destruction and mutation, the true power of the Warp.
 * Exalted Sorcerors - High-ranking sorcerors in the Thousand Sons legion, the Exalted Sorcerors are the equivalent of a Chaos Lord for the sons of Magnus the Red. Their position in the hierarchy of the Thousand Sons is not as fixed as they might want to believe, however. As servants of Tzeentch, they must constantly be on the lookout for the schemes of an ambitious underling.
 * Malignant Plaguecasters - Sorcerors unique to the Death Guard and other followers of Nurgle, Malignant Plaguecasters are bloated horrors, capable of manifesting powers directly related to the God of Entropy and Decay. The magic of the plaguecasters can rust their allies weapons, imbuing their blades with the deadliest of the Plaguefather's curses, or rot the flesh from their enemies bones.

Orks

 * 09053E5F-2875-4B9E-9FC7-0863D75F2973.jpegboyz - All orks generate a gestalt psychic field, known as the Waaagh! This psychic field is what allows the orks to warp reality around them, making their belief into reality. It is because of the Waaagh! that an ork shoota can fire, when in the hands of any other race it is little more than a slab of metal vaguely shaped like a firearm. Weirdboyz (or Wyrdboyz, as they are known amongst Feral Orks) are a special type of Ork Oddboy who possesses the ability to directly tap into the Waaagh!, wielding it like the psykers of other races wield their own powers. Using Waaagh! Energy, a weirdboy can summon effigies of the ork gods Gork and Mork to smite their foes or teleport themselves and any orks around them to another part of the battlefield. However, tapping into the Waaagh! has an adverse effect: it is entirely possible for the weirdboy to take in too much energy, resulting in what is known as the 'Eadbang, where the ork's skull explodes, killing the weirdboy and anyone near him instantly.
 * Warpheads - Orks do not believe in restraint or control. More of something is always seen as better, and as such, more psychic power is seen as better for a weirdboy. Should a weirdboy survive without blowing himself apart long enough for his powers to fully develop, he is likely to push himself further, revelling in the new power granted to him, rather than fearing the risk that comes with using it. These crazed weirdboyz, known collectively as Warpheads, are much more powerful than a regular weirdboy.

Tyranids

 * Synapse Creatures - In the Tyranid Hive, there exists creatures known as Synapse Creatures. These creatures possess a psychic synapse link to lesser tyranids, allowing them to control the swarm in battle. Many synapse creatures, such as the Hive Tyrant or Zoanthrope, are psykers themselves, whilst others, like the Tyranid Warrior, are limited to synapse, and can't actually do any more than that.