A psyker is a being who exhibits the ability to use psychic powers.
Psychic powers can take many forms, from predicting enemy movements to projecting bolts of devastating psychic energy. Some can even aid troops towards more devastating attacks.
The psyker mutation is becoming increasingly common among humanity, presaging the emergence of a new, psychic race. First appearing in large numbers by the end of M22[5b], Psykers represent humanity’s future: the ideal creature into which mankind will evolve, a more powerful, intelligent and capable life form. This new race is still weak, its members lacking the mental strength needed to resist the dangers of the Warp. They are both a threat and a boon to the Imperium.[Needs Citation]
Left unchecked, psykers present a potential danger to entire worlds, and much of the Inquisition’s role is focused on hunting them down. The Imperium ensures psykers are suppressed, tracked down and controlled; those strong enough will be recruited into Imperial service, fulfilling vital roles in the Imperium as Astropaths and Sanctioned Psykers. The weakest are doomed to serve humanity through service in the Adeptus Astronomica, or as nourishment for the Emperor of Mankind.1 The Imperium classifies its own Psykers based upon their level of psychic ability. Known as The Assignment, the levels given indicate the power of the Psyker. An Omnicron level Psyker may only have mild subconsciously activated psionic abilities while an Alpha-Plus level Psyker is capable of snapping a Titan in two.