Psyker

In the fictional universe of Warhammer 40,000, psykers are individuals with some form of psychic ability. They draw their power from the Warp, a parallel universe of psychic energy, and can use this energy for a variety of effects, such as reading minds, foretelling the future, healing injuries, or incinerating foes. Because the Warp is inhabited by daemons, all psykers are at a constant risk of being possessed.

Psykers draw their power from the Warp, an alternate dimension that has multiple properties including enabling Faster-Than-Light travel, as well as being home to powerful psychic entities such as Daemons and the extremely powerful Chaos Gods, which embody the destructive desires of the mortal races (War, Pleasure, Greed, etc.). Human pyskers are vulnerable to corruption from these forces due to their lack of training and evolutionary progress, unlike other species such as the Eldar who have mastered their ability, or have other defenses native to their species. Since the human psykers are vulnerable, they are easy prey for demonic possession, which allows daemons and other threats to enter this world from the Warp to wreak havoc. To combat this problem, the Imperial Inquisition captures pyskers, and either trains them to control their ability, or sacrifices them to the Golden Throne, a mechanical device that sustains the Emperor of Mankind - himself a powerful psyker, and the main defense against the forces of Chaos that wish to destroy humanity.

Sentient beings have a mental connection to the Warp, a parallel universe of pure psychic energy. Those with noticeably strong connections are called psykers. They are able to develop supernatural powers such as telepathy, divination, or pyrokinesis. The Warp is influenced by the thoughts and emotions of sentient creatures, and these energies sometimes coalesce into entities known as daemons and Chaos Gods. Those entities prey on psykers as conduits to the Warhammer "realspace", and may cause psykers to go insane or become demon possessed. Psykers must therefore undergo training and possess strength of will.

Imperial scholars grade a psyker's power using the Greek alphabet, with the most powerful psykers being Alphas. Following them in order of decreasing power are Betas, Gammas, Deltas, and so forth. The median grade for measuring psychic abilities is at Rho and Pi, where no psychic talent is present; the common human is represented by these grades and represents most of the Imperium. Those below Sigma grade are the "negative" end of the scale, and are considered "anti-psychic" and usually referred to as Untouchables, Pariahs, or Blanks. The most powerful psykers, who exceed the grading scale, are considered Alpha-Plus, such as The Emperor of Mankind, while the lowest recorded example is an Omega Minus.

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