Psyker

Psykers are members of any sapient species who possess psychic powers due to a connection to the Warp. Due to the inherent chaotic nature of the Warp, as well as its necessity for long-distance space-travel, Psykers are often considered a necessary evil, individuals subject to a lifetime of risk where even the slightest lapse in self-control may result in a fate worse than death.

Depending on their talent and training, a Psyker may be capable of incredible power often comparable to magic or even psionics of other universes. A Psyker may possess the abilities of telepathy, telekinesis, producing lightning or fire, or even foreseeing the future. However, due to the significant connection to the Warp essential to being a Psyker, they are constantly under threat from the influence of Chaos. Untrained and undisciplined, a Psyker may be driven insane or even killed by the intrusion of Chaos into their minds, they may become possessed by Daemons or even physically transformed into a living portal to the Warp, allowing potentially armies of Daemons to invade realspace.

Human Psykers
The single most powerful Human Psyker to have ever lived would be the Emperor, the founder and ruler of the Imperium of Man, and is often considered to be the most powerful Psyker in the entire galaxy. Possibly the only Humans to have ever rivalled him in this regard would be his son, Magnus, and his lord regent, Malcador. With these exceptions, however, Human Psykers are extremely dangerous. Due in part to the Imperium's immense xenophobia, Human Psykers are often subject to immense prejudice and even killed for witchcraft. Others are either sacrificed to the Golden Throne to keep the Emperor alive in his broken state, or trained and sanctioned by the Inquisition. Sanctioned Psykers are permitted to live and serve the Imperium as telepathic communications officers known as Astropaths, starship navigators or even on the frontlines in the ranks of the Imperial Guard. These Psykers, accepted by the Imperium, are often viewed as something akin to wizards or sages. Depending on their skill, discipline and devotion, Human Psykers may even become Inquisitors.

Astartes Psykers
During induction into a chapter, aspiring Space Marines are screened for psychic potential. Those who possess it are inducted into the chapter's Librarium and become known as Librarians. Space Marine Librarians often make extensive use of their psychic powers for offence, such as igniting or electrocuting their enemies, telekinetically launching or crushing them, or even destroying their very minds. Librarians also serve as commanding officers for their battle-brothers and even as advisors for their own superiors.

Chaos Psykers
Psykers who lack training and discipline often become living conduits of the ruinous powers, easy prey for daemonic possession. Psykers loyal to the Chaos Gods tend to be far more unstable and dangerous than others, often destroying themselves and their own allies through their own lack of self-restraint. Khorne and his followers often demonstrate contempt for Psykers, viewing the use of psychic powers in battle as dishonourable. Tzeentch, on the other hand, is the patron-god of many Chaos-worshipping Psykers, hence why so many of his followers tend to be powerful sorcerers. As denizens of the Warp, Daemons are effectively made from Warp energy, and so naturally possess abilities often associated with Psykers.

Eldar Psykers
Unlike Humans, a species in which Psykers are relatively rare, all Eldar are Psykers. However, due to their collective connection to the Warp and its factoring in the creation of the Chaos God Slaanesh, the Eldar of the Craftworlds maintain highly disciplined lifestyles to ensure strict control of their abilities. Typically, Eldar are forbidden from using their psychic powers, unless unconsciously such as operating their technology, or if they choose to follow the path of the Seer. Seers are exceptionally powerful Psykers and are often looked to as leaders among the Eldar. The most powerful Seers are known as Farseers, whilst those with experience in past military service are known as Warlocks.

Due to the prospect of their souls becoming devoured by Slaanesh after death, most Eldar wear a spirit stone on their person designed to catch and trap their soul if they die while wearing it. This enables the Seers to retrieve the stone and upload the soul into an infinity circuit, effectively a Craftworld's power source and an artificial afterlife. If deemed necessary, these souls may be extracted from the infinity circuit and placed within a wraithbone robot, bringing dead Eldar back to the frontlines in a new mechanical form. While most of these fallen Eldar become Wraithguards, or their finest war heroes become Wraithlords, Farseers may be resurrected in the form of a Wraithseer. These constructs are still capable of unleashing great psychic power even in their new mechanical forms.

The Dark Eldar, due to their collective connection to Slaanesh, are prohibited from ever using their psychic abilities under any circumstances, as doing so would expose themselves to Slaanesh.

Ork Psykers
Like the Eldar, all Orks possess at least a modicum of psychic potential. However, the Orks do not draw this power from the Warp, but from their own subconscious telepathic connection they know as the WAAAAGH. This connection enables the Orks to unknowingly manipulate reality itself based on individual belief, although group mentality greatly enhances the effects. This is exemplified best by the functionality of Ork technology, which is typically made up from whatever junk the Orks could scavenge or 'loot.' Despite being so poorly built they shouldn't possibly be able to function, Ork technology, from guns and vehicles to starships, defies the laws of physics and works as intended because the Orks who build and use them simply believe they should function. This also explains why Ork technology would stop working or even collapse if a non-Ork attempts to operate it.

There exists a specialist role amongst Orks who are considered their species' psykers known as Weirdboys. These Orks draw power from the WAAAAGH of other Orks nearby instead of the Warp. The more Orks are present around a Weirdboy, the stronger but more unstable their power becomes. Should a Weirdboy fail to expel the building charge of WAAAAGH energy within themselves quick enough, a timeframe depending on how many other Orks are nearby, their head will explode and kill every Ork around them (a process the Orks refer to as 'eadbang). As such, Weirdboys often serve in an Ork horde as either psychic support or suicide bomber.

Tyranid Psykers
While the entire Tyranid species is telepathically controlled by the Hive Mind, A few breeds of Tyranid are capable of drawing psychic power from the Hive Mind itself. The most notable type of Tyranid Psyker is known as the Zoanthrope, which is believed to have originated from the Tyranids assimilating Eldar DNA into their genome. While the Zoanthrope's body has largely atrophied, its brain is swollen to a tremendous size and the creature is dependant on levitation for mobility. Otherwise, most psychic ability among the Tyranids is present in the leadership breeds referred to as Synapse Creatures, a term applying to Tyranid Warriors, Zoanthropes, Hive Tyrants and the Broodlord among others. These Tyranids are effectively living transmitters for the Hive Mind's influence over lesser Tyranids, such as Gaunts and Carnifexes. If a Synapse Creature is killed and there is not another to replace it, every Tyranid linked to it will be disconnected from the Hive Mind and become feral.

Psykers of Other Races
A soul and connection to the Warp is required in a species before psychic abilities can manifest. Necrons are soulless machines, and so have no psychic potential, whereas Tau have no Psykers due to their negligible presence in the Warp. However, the Tau may still have access to Psyker auxiliaries from their Nicassar allies. Due to their ability to assimilate the traits of other species through eating their flesh, Kroot may become Psykers through sufficient consumption of Psyker-meat. Kroot Psykers are often assigned as Shamans to their kindreds, but the consumption of Chaos-tained meat is forbidden due to the risks of mutation and corruption it would cause.