The Psykers

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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. This is the harbinger of Mankind's slow evolution into a fully psychic species such as the Homo Ultris Celestialis who will be more intelligent, more capable and ultimately more peaceful than humanity is at present. Unfortunately, this process has been quite uneven and 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 wield. 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 largely banned the use of psychic powers throughout the Imperium of Man save in a few notable exceptions. 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.
Psykers are too dangerous to remain at liberty on the Imperium's myriad world; 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. Humans found to be bearing the psychic mutation, if they are not simply killed, are sent to Kor aboard the infamous Black Ships which endlessly traverse the Imperium.
The Black Ship fleet travels constantly throughout the Imperium. Each Imperial world is visited every hundred years or so by a Black Ship. When a Black Ship nears a planet, its Imperial planetary governor is instructed to prepare the customary levy - a tithe of unsanctioned psykers drawn from the world's population. Once the levy has been collected, the Black Ship's captain makes an initial evaluation of his cargo before proceeding to the next world in his circuit. When the starships are full of psykers they return to Kor and the psykers are transferred to the Scholastia Psykana of the Adeptus Astra Telepathica for evaluation and training before the ship leaves again to continue in their eternal search. It is common for Inquisitors of the Ordo Hereticus or Ordo Malleus to travel on board these ships, as this gives them a good opportunity to investigate a planet's potential for psychically-based corruption. The Black Ships are described as a void that blocks out the light of the stars visually as they pass. The captains of Black Ships are often senior Inquisitorial agents with a ship-based defense force of Inquisitorial Storm Troopers.

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Part of a Storm Trooper legion. These troopers are usually found in black ships travelling around the empire