A Psyker is an individual of any intelligent species in the Warhammer 40,000 universe who possesses some degree of psychic ability or powers. Psychic powers are sometimes referred to as "sorcery" and their users as "witches" in the Imperium of Man and among the Eldar, particularly when such powers are used in such a way that they show no regard for the dangers they may spawn for others or are used in pursuit of the agenda of the Chaos Gods.
Psykers draw their powers from the extradimensional realm known as the Immaterium or the Warp that underlies four-dimensional realspace and is the source of all psychic energy in the universe. As such, psykers, particularly human psykers whose minds are far less disciplined than their Eldar counterparts, are often in danger of possession by the daemonic entities of the Warp, insanity and ultimate corruption by the Ruinous Powers of Chaos.
Psykers of the Galaxy
Most of the intelligent races in the Warhammer 40,000 universe employ psykers on the battlefield; some of these psykers are inevitably more potent than others. There are three exceptions to this: the T'au, the Dark Eldar and the Necrons. The Tau have yet to develop any psykers among their race because they have only recently evolved to fully sentient status. The Dark Eldar, though as possessed of inherent psychic ability as their Craftworld counterparts, fear that the use of psychic powers only further hastens the loss of their souls to Slaanesh. The Necrons no longer possess souls since they transferred their consciousnesses into their necrodermis bodies and became creatures wholly of the physical universe like their former C'tan masters; as a result they no longer generate any presence within the Immaterium and cannot draw upon any form of psychic energy from that realm.
The Imperium of Man
The genetic mutation which grants a human being the ability to draw upon the psychic energy of the Immaterium has become increasingly common since the Neolithic Age. However, what is known is that human psykers were only first mentioned in the historical record towards the end of the 22nd Millennium, making a sudden appearance on almost every human-settled world within a relatively short span of time, not long before the onset of the Age of Strife. This change was the harbinger of Mankind's slow evolution into a fully psychic species much like the Eldar who will be more intelligent, more capable and ultimately more peaceful than humanity is at present. Unfortunately, this process has been quite uneven and even the most powerful human psykers still have minds that have not developed to the point that they can touch the Warp with the level of control wielded by the Eldar Farseers. As such, because of this lack of mental development and Mankind's own still deeply flawed, covetous nature, most human psykers always run a substantial risk of being possessed by the daemonic entities of the Warp and spreading the death and destruction of Chaos wherever they go. It is for this reason that the Emperor of Mankind largely banned the use of psychic powers throughout the Imperium of Man save in a few notable exceptions at the Council of Nikaea during the Great Crusade. Psykers, although justifiably feared throughout humanity's history because of their extraordinary powers and the equally extraordinary dangers they sometimes unleashed on those around them, serve several vital roles within the Imperium today under the exceptions laid out by the Council of Nikaea some ten millennia ago.
Psykers are too dangerous to remain at liberty on the Imperium's myriad worlds; their minds are susceptible to daemonic possession as their strong presences in the Warp can serve as gateways into the material universe for all kinds of Warp entities if they are weak or undisciplined with their powers. Uncontrolled, they can intentionally or unintentionally cause the destruction of entire worlds and have done so in the pre-Imperium past, such as during the terrible days of the Age of Strife. Humans found to be bearing the psychic mutation, if they are not simply killed, are sent to Terra aboard the infamous Black Ships which endlessly traverse the Imperium.
