"He who fights with monsters should look to it that he himself does not become a monster."
- – Friedrich Nietzsche
"We always vilify what we don't understand."
- – Nenia Campbell
A psyker, in the Warhammer 40,000 universe, is an individual with psychic powers capable of accessing the Warp. This is both a blessing and a curse; though psykers are often able to achieve spectacular feats with their abilities, the Warp is a perilous place, and whenever a psyker uses their abilities there is a chance that they will be subject to the Perils of the Warp. Death is one of the milder results of this; extremely unlucky psykers might actually be raped in countless ways by daemons and used as a gateway from the Warp into the physical world. Luckily, in the ranks of the Imperial Guard, where there's a psyker, there's usually a Commissar ready to unleash the power of Blam.
- 1 Types of Psyker
- 2 Psychic Power Hierarchy
- 3 Psychic Disciplines
- 4 Gallery
Types of Psyker
The different races of the setting perceive psykers in different ways.
The Imperium of Man regards most psykers as extremely dangerous, because these are people with scary powers, and the influence of the Warp tends not to let them stay sane for very long, but hey, space wizards are cool*BLAM* HERESY!. There is also the threat of Chaos possessing them and summoning daemons. Thus, the Inquisition hunts them down so they can either be sacrificed to feed the Empra or sanctioned and made to serve the Imperium (under an inquisitor's watchful eye). Such psykers appear as a unit in the Dawn of War games and like to scream incomprehensible one-liners so mind-numbingly loud that they may very well actually make your head explode. Space Marines make their psykers into Librarians: in addition to psychic support on the battlefield, they compile records in the Librarium, serve to assist in battlefield communications, and test their brothers for Chaotic contamination. Imperial Guard Sanctioned Psykers combine psychic powers with Standard Issue Balls of Steel that every Imperial Guard member is issued upon conscription alongside with lasgun. In addition, they have a nasty reputation for being BLAMMED by Commissars. "Rogue Psykers" is the official designation for any psyker who hasn't been collared by the Imperium yet and are often hated by more puritan worlds, in extreme cases routinely being hunted down and killed; but on most worlds being sactioned (assuming you survive and don't mind the side affects) gives an air of authority and orthodoxy to an otherwise hated thing. People still hate you, but instead of treating you like a particularly verbose bit of dung, now you're hated the way tax collectors are hated. Astropaths are beyond such venom, partly because they are less prone to possession and mostly because they are a planet's only way of communicating with the greater Imperium. People still feel uneasy around them, but even Black Templars have to cope with them and make nice faces. However, the Emperor
was is a psyker, so the next time some idiot tells you that Psykers are heresy, use this little fact on their fa--*BLAM* EXTRA HERESY!. Well, actually not, since for an average citizen of the Imperium the Emperor is God, so His wielding godly powers in the glory days before His ascension is actually the core tenet of the Imperial Cult and proof of his divinity. Not so for mere mortal psykers, who are rightly considered unholy and corruptible.
Humanity, being the stars of the show, obviously have arguably the greatest psychic potential overall (because of course they do), with a psychic evolution (which could take countless centuries or even millennia to take place, even if they can survive it) into a fully psychic race which the Emperor himself has said may very well equal or even possibly surpass even the Eldar at their peak, although with the loss of the Webway project even the Emperor admits that this path is pretty much now closed to humanity. Becoming a psychic race doesn't suddenly make you god tier, it just makes you psychic; the Eldar are a full-blown psychic race and they aren't exactly doing brilliantly.
- Like all of his interactions with others, anything that the Emperor says should be treated as a possible falsehood (he's a bit of a compulsive liar and manipulator). His whole speech to Ra about humanity's possible psychic might may well have been just what the Emperor wanted Ra to hear in order to prepare him to make his sacrifice at the end of the book.
Humanity has managed to produce some of the strongest Psykers in the history of the Galaxy but most didn't come into being through natural means:
- The Emperor is obviously the first that springs to mind as he is unquestionably the strongest mortal psyker that has yet been seen, unless you count the Old Ones (Emps could be an Old One masquerading as a human). The question is however: is he even human? There are many theories surrounding the Emperor's origins and his exact nature. Was he born or is he a construct from the Dark age of technology that believes itself to be human? If we take the whole Shaman origin as fact then his psychic might is due to the large number of psychic beings merging their minds into one which is certainly not the norm (strangely something similar happened to Ephrael Stern). Even if we assume he's a single entity that happens to be immortal, there is the whole deal with the Chaos gods which may or may not have boosted his already considerable power (and yes, it is stated that he needed a ship to get there, but didn't need one to get back). The Emperor is clear proof that, whatever he might be, the most powerful psyker in existence has indisputably allied itself with humanity.
- Magnus is a Primarch and should in no way be considered entirely human. He, like all his kind, are artificial beings created through Warp shenanigans and advanced bio-engineering. There is nothing natural about him or his psychic might.
- Malcador is the only truly natural (depending upon what happens with his backstory once it is finally revealed) human standout amongst the big names. He may play second fiddle to the likes of the abominations that are the Emperor and Magnus, but he is still the strongest "human" psyker on the list. Natural humans are never going to be able to compete with the gods of the Warp, but a race of potential Malcadors doesn't sound too shabby, does it?
Human psykers (with occasional exceptions, like Space Marine Librarians) tend to be one trick ponies; if they are telepathic, then that is what they can do - it is very unlikely that they will have any skill in something like telekinesis, pyromancy, or divination. If this was an RPG, then they would be the guys who put all their experience into learning one or two skills until they have maxed them out, but end up having nothing left to spend on anything else. Yes, they can create a very powerful fireball, but that's all they can do. If anyone is capable of "omnimancy" then they are usually so powerful they can brute force powers from all disciplines, or are engaging in rituals and sorceries which is technically cheating, and stupidly dangerous to even masters of the art.
The Squats that grow very old are called "living ancestors". They are extremely rare, and develop psychic powers by communing with the spirits of their predecessors. And you could put them in an egg-shaped terminator armor, then onto a trike, with a bolt-firing cavalry lan-*BLAM* Squats are heresy!
Forces of Chaos
Chaos tends to have...varied opinions on psykers.
- Daemons are made out of the stuff of the Warp, and using psychic powers is shaping and throwing out a little bit of the Warp, so a daemon using psychic powers is the rough equivalent of taking a severed limb or some bones and throwing it as a viable attack. While that sounds pretty fucking metal, Khorne thinks that pulling this bullshit is a sign that you're not manly enough to get into the thick of it and handle the situation on your own, and opinion is shared by his daemons and mortal followers. Not only do they not use psychic powers, but they actively hate any pussified deadbeat who uses them, and they even get toys that fuck with anybody trying to use those powers. However, it should be said that (deeply confused) Khornate traitor librarians exist - Khorne's opinion of them doesn't stop them from idolizing him. They just don't receive his favor, obviously (Unless your name is SSSSSSIIIINDDDRRRIIII). Any psyker who wishes to serve him swears off their powers except for utility like telepathy or to stop other psykers from cheating. Having a cheat code and not using it makes you more honorable, not less.
- On the other end of the spectrum, Tzeentch, being the god of magic, is hugely into psychic powers, and he likes making new ones that do all kinds of mix-and-match effects, though only the best are actually rolled out. That's the official line. He's actually a closeted pyro, and a lot of his powers are burny or...do weird shit. Being the patron of psykers, if people get Tzeentch's attention because they're scholars or David Xanatos, they'll eventually develop psychic power whether they like it or not. Due to higher exposure to Warp energy via psychic powers and serving the god of change, Tzeentch's minions, and more often, their minions have the highest rates of mutation and turning into Chao--er, tentacly gribblies. Ahriman, possibly the greatest of Tzeentch's servants barring Magnus himself, was so powerful a Psyker that he was able to defeat Ulthwe's most venerated Psyker, Kyaduras the Anchorite, with ease, and, in the same battle, also casually defeated Yvraine, the Yncarne, and the Visarch.
- In the middle, Nurgle likes any psykers that spread his gifts around, but doesn't really go out of his way to promote them in his followers either, probably on account of his direct opposition to Tzeentch. Needless to say, his powers all revolve around inflicting people with horrible diseases or boosting their endurance to unnatural levels.
- Similarly, Slaanesh thinks Tzeentch is a giant nerd, whose overdevotion to his nerdy hobby of inventing new psychic powers will never get him laid. We think it says this because Tzeentch
refuses to sleep with it (it almost certainly likes his tentacles and crotch mouth)is afraid he might catch Warp chlamydia, and he's just not willing to give Nurgle that kind of satisfaction. Slaaneshi psykers emphasize powers that make the hurt feel good, or have effects that look like the target might have snorted the psychic power off of a stripper's ass.
Being able to combine psychic powers, sorcery, and the favours of the Dark Gods easily makes the psychic followers of Chaos amongst the most powerful and dangerous individuals to have ever existed within the 40k setting.
The Craftworld Eldar are a strongly psychically-attuned race. While the regular, off-the-street Eldar will restrain themselves from using any psychic powers beside basic telepathic communication and the ability to use their psychic technology out of fear of being mindraped by Slaaneshi daemons, they can join the Path of the Seer and train to better control their innate abilities. Eldar who go on this path with previous experience as Aspect Warriors become Warlocks, while the most powerful seers can become Farseers and leaders of their people. They're ridiculously powerful psykers because of that deep imprint in the Warp to draw power, combined with their Spehss Ehlvan minds and willpower. They just need to be fucking careful and avoid unwanted attention unless they like tentacles.
The true capacity and depth of Eldar psychic abilities tend to be rather watered down or just plain ignored outright, due to the demands of the plot which is almost always told with the Imperium as the protagonist of the story. Within most stories the authors will tend to use the "M. Night Shyamalan" style of writing, which means having the Eldar Farseers getting a vision of events that are going to happen in the future and set out to stop it, only to discover that they either had it wrong the whole time, or they themselves brought about the very event that they wished to prevent; all the while the authors are hopping around shouting "WHAT A TWIST" at the top of their lungs. Due to the vast focus put upon the forces of the Imperium it is not a surprise that there is such an abundance of named human Psykers running rampant across the galaxy, but remember that humans only produce an exceptional psychic individual once in a blue moon; those humans that prove themselves strong and stable enough to join the ranks of the Marines as Librarians are a rare breed but more than prove their worth over multiple centuries of service.
By comparison, every Eldar citizen can, if they wish to, potentially become a Farseer; although no Farseer will ever manage to reach the same level of power as the likes of Magnus it's not really a fair comparison as he really isn't human and did not come into existence through natural means. In a slightly darker turn of events, where the likes of Spiritseers, who are able to commune with and manipulate the souls of the Eldar dead, the Ynnari actually amplify their existing powers through absorbing the souls of previous Eldar, which many Eldar among the Craftworlds consider to be horrifying. GW's "fuck what the Xenos are supposed to be able to do; the Imperium and Chaos are the stars of this setting" rule is seen many times in the story itself, with powerful Eldar Psykers like Eldrad very rarely performing impressive feats within the stories they are featured in, and you are just told that they are very powerful before they are inevitably defeated by the glorious human protagonist (Loyalist or otherwise). Eldar Farseers, in particular, suffer from the "Worf effect" more then most, despite often being heralded as great seers they have a terrible track record within the storyline itself, very rarely succeeding in any of their endeavors. An important thing to keep in mind is that The Black Library writers love to add their very own super duper special human snowflake Psyker into the story because the "Rule of Cool" demands it, with many in the lore being attributed with the ability to destroy Titans and armies. Farseers on the other hand are often shown being incapable of even defeating a single Space Marine, let alone a tank, army or Titan, and Farseers are, explicitly, meant to be the most powerful Eldar Psykers in the galaxy. However, this could simply be either a way of keeping it so they don't win by default due to knowing exactly how a battle will go or (much more likely) Black Library incompetence making it impossible to get a remotely accurate depiction of what their powers can do.
The 8th edition codex seems to have made an attempt to address this situation. Whilst all Eldar walking upon the Witch Paths are capable of mastering many different physic disciplines throughout their long lives, they tend to avoid developing their more destructive powers, instead focusing all their efforts on developing their more “helpful” and “creative” powers; such as the Bonesingers, who use their powers to create and then craft Wraithbone. This goes well with the Craftworlder’s mindset when it comes to avoiding their darker more destructive sides.
Those who find it easier to master the more destructive disciplines tend to be those who have walked the Path of the Warrior and have already had their more violent impulses tested. These individuals will become Warlocks, and can make use of their more destructive powers upon the battlefield without putting those around them in danger; as they already have their War Masks in place, to help focus them and to separate their warlike selves from their normal selves.
Farseers will unfortunately still suck in any story that they are featured in, but they are not supposed to be just weapons, they are guides and advisers who help guide the blade, not swing it themselves. This, of course, doesn’t help the fact that they are still going to be treated like a bunch of red shirts, that will inevitably fail at every single opportunity by the writers at GW.
The Eldar make use of Rune Stones that act as conduits through which they can manipulate the psychic energies of a the Warp without actually having to interact with the Warp directly; though this does limit their overall psychic power level it does provide them with a much safer and controlled way to access the energies of the Warp. The Rune stones also act as psychic fuses burning out and cutting the link to the Warp should an Eldar try to tap into too much power before they are ready. As they develop their control and skill the Eldar psykers will eventually be able to use a far greater number of stones, which allows them to tap into far greater pools of power.
Eldar tend to be the Swiss Army knife of psykers; being able to learn many different psychic disciplines. If this was an RPG then they would be the guys who spread their experience out over a wide range of powers and abilities. They don't tend to max out their powers, but instead end up with a much wider skill set to call upon.
Harlequins are also psychic, which really comes out in their shows, and although only the Shadowseer can use powers in a fight, other Harlequins do enjoy the benefits of psychic senses and silent telepathic communication.
The Dark Eldar, however, are psychically atrophied compared to their Craftworld bros. They have to avoid using psychic powers due to Slaanesh's "I own your soul" gig, and Dark Eldar using psychic powers are reasonably feared as bait for snusnu-with-a-Daemonette. They can use psychic artifacts, like all those crazy living swords and Haemonculus toys; the only psychic power they use is their infamous emotion-vampirism ability that can replenish their bodies and souls. And kamehamehas, for Mandrakes. They do have extra fun torturing, killing and then weaponizing psykers though.
We don't have any crunch and not a lot of fluff on these guys, but presumably they're psychically attuned, too, given their ability to maintain a healthy relationship with their world-spirit and beasts of burden.
The Tau race is incapable of producing psykers, due to having almost no presence in the Warp. The good news for them is that they don't have to worry about Bad Things, such as any members of their species spontaneously turning inside out from the anus and summoning an army of monsters that will fuck, kill, and eat people, (not strictly in that order). The bad news is that they have no way to natively develop psychically based technology, so their FTL travel is glacial, albeit a good deal safer than an actual Warp jump, and their FTL communications options are limited to the Space Pony Express. It should be noted that some of their vassal races have psychic abilities, including their own Navigator analogues. It is known that they actually locate, isolate and study human (and probably other species') psykers, but as the Immaterium is too irrational and far beyond their science they have had little success in analyzing the reason behind Gue'La suddenly going crazy and
summoning teleporting armies of homicidal hellspawn aliens out of thin air. Of course, following what happened to the Fourth Sphere Expansion, they now have little reason to stay ignorant of those things that come from nowhere...
- Funnily, the Kroot have psykers (they have a diverse diet) and understand a great deal about the Immaterium. They even have some crude warp vessels, usually with purchased or stolen xeno tech in it.
- The Nicassar are another auxilia race that just happens to have psykers of their own, though their potency when compared to the big boys of the setting hasn't really been examined. Regardless, they know enough about the Warp and about Tech to help develop the Tau's near-FTL technologies.
The Necrons no longer have souls, so they have no presence in the Warp, and therefore can't manipulate it with their minds. And yet, they have managed to conquer the Warp with technology. (Egyptian Skeleton Zombie Robots, Yeah!) They keep fucking up the Warp and psykers with the gloom prisms they mount on their spiders, pylons (sounds familiar...), or even giant city-sized mechanisms that cut entire planets from the Warp. One in the Word Bearer novels even had...this thing, about the size of a resurrection orb, which blanketed an entire system in a "psychic black hole." Problem, daemons?
Before the 5e fluff changes, they also had Pariahs, specialized soldiers made from Blanks that acted like Culexus assassins.
The Orks are all passive psykers, or rather emanate their own gestalt "psykik" energy in the power of the WAAAGH!, though they're mostly unaware of this. A large number of Orks in one place will slightly alter reality to suit their wishes, turning jam-prone, dirty blunderbusses into fully-automatic rifles of rape. Weirdboyz are the only ones who can tap into it actively to manifest psychic powers, which have an amusing tendency to make Orks heads explode with absolutely no warning. As said above, the Orks draw psychic energy from each other, not from the Warp, which makes them virtually immune to Chaotic influences(but not completely immune), although they do have a pretty serious presence in the Warp in the form of Gork and Mork (or maybe the other way round).
The Tyranids are innate psykers, and their collective power goes into the Hive Mind, which is what the big bugs tap into to manifest psychic powers rather than the Warp, the same way a Weirdboy would tap into the WAAAGH!. Only a few bugs are capable weaponizing the Hive Mind's psychic energy, the Zoanthrope being one such option for Tyranid psychic support. Few 'Nid species beyond the Zoanthrope are capable of firing tank piercing mind-lasers. Instead the other big bugs use its powers for buffs and a form of control called "synapse", which keeps the Tyranid horde from going feral and killing everyone and everything they see. If the Hive Mind gets driven away from the horde or the synapse bug gets killed, the synapse link collapses and the 'Nids turn feral. Also, before Genestealers got retconned into being Tyranids, Genestealer Magi DID use the power of the Warp to power their abilities and could dedicate themselves to Chaos. There's an image of a Khorne Magus in one of the 2nd edition books, though that is nonsensical in itself given Khorne's view of psykers.
Psychic Power Hierarchy
The Imperium of Man uses a 24-point scale (with gradations named for the letters of the Greek alphabet, for some reason; there are 26 points counting the "top" and "bottom", both of which are used to classify entities beyond the scale's ability to measure) called The Assignment to determine the comparative psychic power of an individual. Unfortunately, this gets very confusing in the fluff, where Alpha-level psykers seem to be a dime-a-dozen and show up all over the place, and established characters that are canonically high on the scale are completely overshadowed by some other character. All of this "logically" leads neckbeards to reach the flawed conclusion that their favourite character on the tabletop MUST be Alpha level or greater.
Furthermore, it is canon that a psyker's place on the scale can change over time as his or her powers either increase with practice or decrease due to atrophy from lack of use. So generally, the fluff is quite useless for determining who is greater than whom; without the evidence of a direct psychic battle between any two comparing how they match up, it is pretty pointless to use this method.
Lastly, this is an Imperial system, and likely should be used only to assess humans. While chances are, someone in-universe has looked at a wyrdboy and gone "Theta-ish", you shouldn't since it's comparable to using metric tools on imperial sizes. Which is to say, it's annoying, inappropriate, and probably a crime according to both the Mechanicus and Administratum. So don't do it.
Positive Psychic Levels
- Alpha-Plus psykers do not represent the top of the scale, but are completely beyond the scale. In fact, the scale theoretically continues upwards beyond Alpha-Plus into truly god-like tiers of power, although such individuals have never been encountered. Even so, Alpha-Plus individuals operate on a completely different wavelength from normal people and are owners of completely alien mindsets. In terms of power, they are akin to weapons of mass destruction, as in theory there is nothing that a trained Alpha-Plus psyker cannot accomplish through force of will. There is a tacit assumption that this category contains the Emperor and his "notably-psychic" Primarchs (especially Magnus the Red in his daemon Primarch form), but even he admitted there were limits on his power, specifically that one could not be both all-seeing and all-powerful at the same time. No attempt is made to distinguish power levels among members of this grade, because they're all so absurdly strong there's no point in quantifying it. To put it in context, an emergent Alpha Plus psyker - not one in fully control of their abilities - was able to destroy a starship, freeze time, put it back to together and resurrect it's crew from the dead before removing itself permanently from the physical universe in an act of unparalleled self control.
- Alpha / Beta
- Exceedingly rare and dangerous. The Imperium believes that human beings have not sufficiently evolved enough to contain Beta and Alpha levels of psionic talent without going completely bat-shit mental, though exceptions do exist. Arvann Stern is confirmed to be of Beta level, and he's not even the most powerful psyker in his chapter. Most psykers of this power level are considered too dangerous to control and are likely to be sacrificed to power the Astronomican, while very unlucky Alpha psykers may instead find Imperial service in the Ordo Sinister, meaning they'll get turned into screaming batteries for Psi-Titans. The unluckiest Alphas and Betas get to be Grey Knights, and fight against daemons until a painful death in battle. The luckiest ones become specialized Astropaths whose predictions of the future are used by the Adeptus Custodes to find threats to Terra.
- Gamma / Delta
- Approximately one out of every billion human births will produce a Delta- or Gamma-level psyker, who, for the Imperium, represent the "useful" upper limit of psychic potential without being too dangerous to let live, though it's up to the Inquisition to be the ultimate judge of that. Most psykers of this power are either slain or possessed if they are not discovered quickly by the Adeptus Astra Telepathica. The Inquisitor Gideon Ravenor was believed to be a high Delta-level psyker.
- Epsilon / Zeta
- Epsilons and Zetas represent relatively powerful psykers, who are strong enough to be considered threats to security by the Imperium.
- Eta / Theta / Iota
- Iota represents the first "true" psykers who are able to manifest and control psychic abilities with a modicum of training. Anything below this point is not considered a "true" Psyker.
- Kappa / Lambda / Mu / Nu / Xi / Omicron
- People who can manifest psychic talents either unconsciously or so subtly as to be beneath the notice of everyone but the Inquisition, such as performing minor "magic" tricks, divinations, or other weird occurrences generally dismissed as good luck. Most of the time they are quite useless, but still represent a danger since they can be possessed. Although the Inquisition is too busy to sort out everyone at this level, the Imperial public is conditioned well enough to spot and ostracize these witches if the truth about them ever comes to light in their societies. Those that do get picked up by the Black Ships are generally considered not worth the risk of training and almost always receive a one-way trip to the Golden Throne to have some sort of use wrung out of them.
- Pi / Rho
- Run-of-the-mill humans, neither psy-active nor psy-inert.
Negative Psychic Levels
- When you start descending below the scale of humans, into the "negative" side of the scale, you start going psy-inert. People at the Sigma/Tau level would be able to perceive and be injured by an incoming fireball but may not be able to discern that it came from an unnatural source.
- Upsilon / Phi / Chi / Psi (ironically)
- These are individuals with varying degrees of immunity to psychic powers, though not to the same extent as a true Blank or Pariah.
- Usually referred to as Untouchables, Pariahs, or Blanks. Not only are they immune to psychic powers, but they can also inhibit the powers of psykers around them and/or neutralize psychic effects on an area around them. The Imperium often employs Blanks as an Inqusitor's servants, while the Sisters of Silence are a trained order of Psyker-huntresses that are all Blanks. The Eldar, believe it or not, also produce the occasional Blank, and these become the infamous Solitaires of the Harlequins. True Blanks have such a psy-negative effect that even normal people can sense it, and it manifests as an irrational loathing for the Blank, though in some cases it can be offset by either a good personality (Alizebeth Bequin) or an odd, but explainable character trait (Gunner Jurgen). For psykers however, Omega-level persons are physically and mentally painful to be near.
- The fluff is a bit inconsistent on the exact limit of a Blank's ability, but that's fine, because in Dark Heresy, Blanks exhibit a broad range of effects which can be different from person to person, much like high-level psykers can have different powers and disciplines. It can also be trained and manipulated, as in the case of the Sisters of Silence. Others can train themselves to actually benefit from the psychic power they suck from their surroundings (having psykers or even normal living beings in those surroundings really helps), which they use to empower and invigorate themselves.
- There have, however, been a few occasions where a Blank's abilities have been overwhelmed or even permanently shutdown. Bequin's blankness was overwhelmed by the warp corrupted psi-sentience of a Chaos Titan and Wystan Frauka's blankness was gradually eroded away over time until he was able to be possessed by a psyker. In Gathering Storm, when Khaine's Gate broke, all the strapped and likely sodomized Blanks that were chained to it and had been screaming for the past few decades either exploded into ludicrous gibs or got outright possessed. Alpha Plus Psykers like the Emperor can probably brute force their way through even the most unnaturally powerful blanks.
- Strictly limited to Culexus Assassins, who are given specialized equipment to maximize the effectiveness of their Blank abilities.
When you get further into the details, not only is sheer psychic potential a factor to consider in determining how powerful a psyker is, there are other things to consider when trying to figure out mechanically how one differs from another.
This is easier to do outside of the tabletop setting, which focusses on a d6 scale, where a minor difference in a single statistic can create one power tier an order of magnitude more effective above another.
Level of Control
The Willpower, discipline and training (read: control) of a given individual holds almost about as much sway in saying who is more powerful than who. This is most clearly represented in the RPG rules, but often shows up in the fluff particularly in instances where powerful but neophyte psykers are only coming to learn their powers (such as Da'Kir of the Salamanders, or Zahariel of the First Legion) A lot of commentators forget this and focus purely on psy-rating when attempting to compare psychic ability. Just because a psyker has a high psy rating, if their ability to control their powers are low then they are less likely to successfully manifest but are more likely to create a powerful effect when they do, this is in fact part of the reason why the Imperium considers them to be so dangerous. Inversely, a low-mid rated psyker with a strong grasp on his abilities is more likely to successfully manifest and is also more likely to be able to resist/counter incoming psychic effects.
Interestingly enough, even non-psykers with enough discipline and mental fortitude can be trained to resist psychic powers, yet does not mean they are necessarily psychically blunted themselves. Typically this ability comes in the form of intense indoctrination or rote training, or from sheer force of belief and faith to literally "Deny the Witch".
The second thing to consider is the whims of the Chaos Gods, (particularly Tzeentch). Sorcery is entirely separate from Psy Rating/Assignment and even a non-psyker with the correct rites and incantations can pull off similar (if not the same) manifestations as a true psyker. Back during the Horus Heresy, Erebus (and 30k Word Bearers with Burning Lore) is an example of this. Furthermore, although Khorne himself hates psykers and sorcery, the Daemonancers among his own followers do have access to rituals for summoning daemons, otherwise they would be solely reliant upon tears in the warp for receiving daemonic assistance. These ritualistic functions may have been carried over to 8th edition where every chaos character now has the ability to summon daemons (though this could just as plausibly be the champion calling out to his God and being rewarded with assistance while in a location where the realspace/warp barriers are thin or have already been broken)
A true psyker with access to sorcery can increase his power levels even higher and can easily outstrip more powerful psykers than himself, though Sorcery comes with its own perils and generally leads to Damnation and Corruption (no wonder big Emps put his foot down at the council of Nikaea)
Furthermore, certain favours gifted from the Gods also increase psychic output. It used to be that in days gone by, the Mark of Tzeentch used to enhance psychic potential on the tabletop. Nowadays it doesn't, but in the RPG it still does. But obviously once you've been marked by a God of Chaos, you're in their pocket forever, and you best do your utmost to keep them happy or you end up becoming that-which-shall-not-be-named.
Separate from either issue is the fact that Psykers can theoretically increase their assignment rating for a brief instant, drawing surplus energies from the warp and "Pushing" their powers even further than should be possible, basically taking a gamble and temporarily adding MOAR power to their manifestation.
In both the rules and the fluff there are common instances of psykers being able to boost themselves and create larger effects or to overcome more powerful opponents. This was partially illustrated with the 7th Edition ruleset and the shared Warp Charge "Pool", where low level psykers could throw all available warp charges into a single power and overcome the abilities of a higher mastery opponent, albeit at considerable risk of Perils of the Warp.
However, not all psykers have the ability to do this to the same degree, and although not adequately shown on the tabletop such rules were presented in the RPG settings, which broadly fit the established fluff:
- Bound Psykers that is to say, nearly every Imperial psyker who has undergone a Sanctioning process [Including Librarians] or Soul-Binding; can only push themselves so much and they are at greater risk of something bad happening (ie: Perils of the Warp) when they do so.
- Unbound Psykers (Rogue Psykers / Sorcerers) on the other hand can push to even higher degrees of power but shit WILL hit the fan and can almost guarantee their manifested power will come with some form of unintended side effect when they do.
- Daemonic Psykers who attempt to push their powers can go yet higher, but are considerably more likely to cause dangerous psychic phenomena. However since their mere existence on the material plane can be considered a dangerous psychic phenomena they are less likely to be bothered by it unless it is a straight up Perils of the Warp.
See its page.