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.
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 , the Dark Eldar, and the .
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).
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