Council of Nikaea
Following the Emperor's departure from the Great Crusade, a conclave was called to determine whether or not Librarians should continue to be used. With representatives summoned to the Council of Nikaea (often remembered as the Trial of Magnus the Red), both sides pleaded their cases. In the end the Emperor, fearing that Magnus was being way too arrogant and reckless in his pursuit of power and in serious danger of Fausting himself (which is exactly what
ended up happening already had happened!) decided that Librarians posed too great a risk and ordered the Legions to disband their Librariums, return to their battle companies, and never use their psychic powers in battle again-- an edict Magnus ignored. Well, actually, an edict that quite a lot of the Primarchs either totally ignored or later broke. But Magnus broke it harder. The White Scars simply ignored it, but the likes of Dorn and Vulkan implemented it straight away.
Hear now the words of my ruling
I am not blind to the needs of the Imperium, but nor am I blind to the realities of the hearts of men. I hear men speak of knowledge and power as though they are abstract concepts to be employed as simply as a sword or gun. They are not. Power is a living force, and the danger with power is obsession. A man who attains a measure of power will find it comes to dominate his life until all he can think of is the acquisition of more. Nearly all men can stand adversity, but few can stand the ultimate test of character, that of wielding power without succumbing to its darker temptations.
Peering into the darkness to gain knowledge of the Warp is fraught with peril, for it is an inconstant place of shifting reality, capricious lies and untruths. The seeker after truth must have a care he is not deceived, for false knowledge is far more dangerous than ignorance. All men wish to possess knowledge, but few are willing to pay the price. Always men will seek to take the short cut, the quick route to power, and it is a man’s own mind, not his enemy or foe, that will lure him to evil ways. True knowledge is gained only after the acquisition of wisdom. Without wisdom, a powerful person does not become more powerful, he becomes reckless. His power will turn on him and eventually destroy all he has built.
I have walked paths no man can know and faced the unnameable creatures of the Warp. I understand all too well the secrets and dangers that lurk in its hidden darkness. Such things are not for lesser minds to know; no matter how powerful or knowledgeable they believe themselves to be. The secrets I have shared serve as warnings, not enticements to explore further. Only death and damnation await those who pry too deeply into secrets not meant for mortals.
I see now I have allowed my sons to delve too profoundly into matters I should never have permitted them to know even existed. Let it be known that no one shall suffer censure, for this conclave is to serve Unity, not discord. But no more shall the threat of sorcery be allowed to taint the warriors of the Astartes. Henceforth, it is my will that no Legion will maintain a Librarius department. All its warriors and instructors must be returned to the battle companies and never again employ any psychic powers.
Woe betide he who ignores my warning or breaks faith with me. He shall be my enemy, and I will visit such destruction upon him and all his followers that, until the end of all things, he shall rue the day he turned from my light.
Dude could give a speech.
Unfortunately this seemed to be one of those decisions that the Emperor didn't really think through. It added yet another layer of bullshit on top of his fear of the Warp and psykers in general, a group of people who, had they been able to ply their mind-bullets freely, would presumably have been awesome weapons against the traitors when Heresy time came around. This is something that the books and stuff seem to say a lot, but it's kinda... well... it's hard to say how true that actually is/was.
At Istvaan, psykers wouldn't have made any difference (3 legions vs 8 is a bad day for the good guys), and the balance of forces was so extreme that just knowing ahead of time wouldn't have helped. At Calth they might have made a difference. All of the former Ultramarine Librarians had bad dreams in the days before the treachery so maybe they might have seen what was coming, but they may not ever have been able to convince Roboute Guilliman of impending treachery. Remember this was happening just after Isstvan V and communication was moving slowly thanks to the Warp Storms, on top of the fact that Marines fighting each other was totally alien to them. Fluff writers expect you to disregard the fact that the Wolves had already massacred two other legions and tried to do the same to the World Eaters. They would have really been struggling to deal with the magic stuff because it was totally new and a weird combination of technology and warpstuff. Oh, and of course, practically no one who wasn't on Guilliman's flagship or with the 4th company even knew what the problem was. So it seems really unlikely that Librarians would have magically made the Heresy all better.
TL;DR It depends on how numerous and how powerful the Librarians in question were, but they would've probably had a minor impact at best.
In any case, the Edict of Nikaea stood for a year or so at most, and it was ignored or abandoned by nearly all the legions. Because it was stupid.
Why Nikaea really happened?
"The enemy fears the warp as much as they plunge themselves into it"
- – Leman Russ - prior to the Siege of Terra
The root causes of the council of Nikaea are described across several books in the Horus Heresy series. Ostensibly everyone acknowledged the importance of psykers in the Imperium and within the Astartes, but Leman Russ and Mortarion both maintained that the Thousand Sons - and by reflection the Librarians in general - were in danger of becoming Sorcerers and had to be stopped.
Mortarion had already been told that the Emperor planned Nikaea about 70 years before it happened which means that the result was rigged before it even started,and shows that the Emperor's intention with the Nikaea edict really was to prevent abuses of sorcery and further temptations from unclean sources, thereby eliminating the use of all psykers except for soul-bound Astropaths and those exceptions in Russ' Legion (see below) until the Webway Project was completed.
But considering Nikaea in retrospect, Russ (and most of the other Primarchs knee deep in daemon filth for that matter) had come to realise that the loyalists had been maneuvered into ridding themselves of an important weapon that could have been used against the enemy in the war which followed; guns and bullets could only do so much against an unnatural enemy against whom blades and symbols fared much better (symbolic relation to daemon summoning causes swords and fire to be highly effective against them, perhaps this is even why the Salamanders were a major target at the beginning of the Horus Heresy). While Russ was still proud of what he did at Prospero (because deciding to follow Horus's suggestion instead of the Emperor's order is something to be very proud of) and firmly believed Magnus deserved everything he got, he acknowledges that Horus changed the order from "capture" to "kill" because the traitors genuinely feared Magnus' sorcery and its capabilities (especially if Magnus and co became faithful in the Emperor), which would have helped the loyalists win the war via curbstomping all their nifty daemon buddies and ruining their new Warp powers; though Russ also points out that Magnus could have just as easily become a second Horus because no one knew which way his path would take. Therefore sweeping such a powerful figure from the board early makes a degree of strategic sense, and with hindsight Magnus turned traitor anyway.
Ohthere Wyrdmake and Leman Russ claimed repeatedly that the Rune Priests were not using the power of the warp and were instead harnessing the power of "Fenris" so that their Priests were somehow different from the Librarius. Just because you had to be a psyker and had to use your psyker powers to do Rune Priest shit doesn't mean it's psychic. Duh. So when Russ later arrived at Terra prior to the Siege he proposed returning to Fenris to have his Priests divine a method for defeating Horus, claiming that the only way to defeat their enemies was to use their own power against them. Rogal Dorn (at that point the only Primarch who had absolutely adhered to the result of Nikaea) callas Russ a hypocrite because of it. Keep in mind that Dorn is always very certain to tell the absolute truth so far as he knows it. He no doubt understood Leman's own beliefs and arguments... and dismissed them as bullshit.
But if Russ and the Emperor supposedly understood so well the power and the corruption of the Warp, why not simply ask the Librarius to reform similarly to the Rune Priests? The White Scars already had a similar - "older" as Russ put it - tradition that emphasized limiting the amount of power drawn from the source, so why not have the entire Librarius follow the same principle? Russ does privately admit to Malcador that perhaps Dorn was correct and that he really was a hypocrite for calling for the censure of others while flouting the rules himself. Malcador points out that the Emperor always made exceptions for Russ because his purpose was singular - fighting other Primarchs, explicitly pointing to those they do not name, then Horus and the Traitors - and that the Emperor trusted Russ to carry out that duty where no one else could. In part due to these contradictions and the special treatment he received, Russ resolved to have a long examination of his Rune Priests and their relationship with the warp after the war was completed.
Why let the Thousands Sons simply go by their way without any supervision when Custodians were attached to the Word Bearers for arguably far less? Because Lorgar had already displeased the Emperor and been punished with the razing of Monarchia while the proclamation at Nikaea was simply a warning. Magnus was not the only proponent of the Librarius and they were left to police themselves with the Chaplains being instituted to keep an eye on them. Following the Prospero clusterfuck, squads of Space Wolves were attached to all of the Legions to ensure their compliance with the Edict anyway (which raises the question as to what Russ & Co were hoping to achieve because if it came down to it, a handful of regular space puppies vs. a Primarch...)
This most likely boils down to whether the Emperor knew who was going to betray him and when, especially considering the first move in the great game the Emperor plays against Chaos is the corruption of "The Lord of Hearts" by the "Chosen" which happened three years after Nikaea (at Davin). It has already been established that the Emperor's foresight was not infallible and the fate of each Primarch was malleable. Since the Webway project relied heavily upon Magnus operating the Golden Throne, if the Emperor had any idea then of how bad it would become then why continue with the project? And if there was any need to single Magnus out for further censure why not drag him to Terra in chains to be kept under lock and key until his powers were needed? ...oh right.
Whatever popular opinion on the subject is, the truth of the "trial of Magnus the Red" is that the arguments of said Magnus at the Council of Nikaea were quite childish, being "inspired" by the allegory of the Cave of Plato but having its significance completely subverted into a progressist and atheistic nonsense. Originally, the Cave of Plato was an allegory on the metaphysics of ontology and about the initiatic endeavour of anamnesis (ἀνάμνησις), which is operated by the mind (νοῦς) as part of the philosophical process of reaching into the realm of forms known as the Hyperuraniôn (which, in 40K, would count as the Immaterium), to acquire knowledge and perfect society (this is basically the only aspect of the Cave of Plato which goes along with the Cave of Magnus version). According to Heraclitus, "everything flows, nothing stands still" (τὰ πάντα ῥεῖ καὶ οὐδὲν μένει); it is the flux of Becoming, and also the inspiration behind the alchemical / chemical tenet "rien ne se perd, rien ne se créer, tout se transforme" of Lavoisier & Anaxagoras. Since of the Indo-European Tri-Functions, the mechanical and organic aspects proper to the lowest Function, Epithumia (ἐπιθυμία), do not survive death but are obliterated, the "reincarnated" elements linked with the two other Functions have to be re-learned from experience - or rather to be remembered, since in theory they were already there - so as to leave the stream of Becoming and step into the one of Being, unlike those who are just obeying orders or bodily impulses. In other words, the effects of the Rubric of Ahriman, turning Marines into slaves without any spiritual function left and only a shadow of bodily one, may be a parody rewarding / punishing Magnus' foolish revamp of the Cave of Plato; likewise for the Burning of Prospero and destruction of Tizca. Indeed, Anamnesis was basically the lifestyle of the immortal Shamans before Chaos energies infusing the Warp eventually caused them to make the Emperor.
Loyalist Response to Nikaea
- The Ultramarines followed the Emperor's command to the letter, disbanding their Librarius and returning their Librarians to the battle companies. In one of his brighter ideas, Roboute Guilliman himself stated during the Battle of Calth that the Edict of Nikaea needed to be overturned as soon as possible, even deducing that the Edict's timing was so convenient that it could only mean that someone had arranged it. Thus when he thought that Terra and the Emperor had already fallen he decided to build the Imperium Secundus in a manner he thought sensible and kept his Chief Librarian around as an advisor (much to the chagrin of a Space Wolves detachment deployed on Macragge to "ensure" Guilliman's loyalty).
- Without a Primarch, the remaining Iron Hands mostly fell in with the Ultramarines and basically did whatever they were doing.
- Lion El'Jonson re-introduced the Librarians as soon as he discovered that the traitors were using daemons, presumably knowing what shit was going down from his experiences on Caliban and even going so far as to punch the living fuck out of a Chaplain who reminded him that it was going against the Emperor's Edict - as in, literally punched his head off, although said Chaplain was about to execute one of the Librarians for using his psychic powers against daemons and refused to back down (and it was an accident anyway). The Lion felt terrible about it as he had meant to hold back but was a little too pissed to control his strength well enough.
- Rogal Dorn locked all his Librarians up in the Phalanx and refused to even let them rejoin their battle brothers as line-troopers. However, when Nathaniel Garro snuck his way on board to try and rescue them on Malcador's orders, they seemed happy enough where they were. Apparently all the Imperial Fists need to feel happy is thick walls around them. Dorn explained that they were exactly where they needed to be, presumably for keeping them in reserve for when the Imperium really need them. What happened to them afterwards is (as of yet) unexplained. The reasonable assumption is that they were held in reserve for the Siege of Terra, possibly on the advice of Malcador.
- While Sanguinius desperately tried to uphold the Emperor's word, his own former Librarians met in secret and proved their worth by saving the entire Legion from the daemonic precursor of the Black Rage at Signus. So he kept them afterwards. Nassir Amit murderfuckraping the Space Wolves detachment supposed to prevent this so hard they died to death unwittingly helped this along. Maybe. They weren't stupid so they'd hopefully have been more likely to have been like "Okay, well, this was a super-extreme situation and the Emperor would surely prefer to have Sanguinius and his legion than the Edict be kept... we didn't see anything."
- Vulkan also tried the Sanguinius approach. However, there were barely any Salamanders left after Isstvan and by M41 they looked back upon Nikaea as an ancient and archaic edict that had no place anymore.
- Corax was a latent psyker, but voted against at Nikaea. Following a personal encounter with the Gal Vorbak, Corax did a 180 and began advocating using Librarians, nearly getting into a shooting match with the Custodes over it. We don't know if they reinstated their Librarians but it seems extremely likely that they did after the Alpha Legion attack to disrupt/steal their cloning tech (which Librarians could have detected).
- The White Scars (at least those that got the message) just kinda shrugged and twiddled their mustaches and did their own thing same as ever, including keeping their Stormseers around. Since it was also suggested that the Khan himself was a latent psyker (though he favoured using his sword instead) on top of his total disregard for authority and support for the Librarians this shouldn't be that much of a surprise. If nothing else, he could fall back on the excuse that he wasn't at Nikaea at the time the Edict was made and could therefore plead ignorance. His "stormseer" who spoke for the Scars at Nikaea simply privately refused to accept it and vindicated himself shortly after by exploding a daemon with lightning. His psychic powers (and those of the one surviving loyalist from the Thousand Sons) later saved the entire Legion during the Horus Heresy.
- The Space Wolves supported the banning of Librarians but kept their own afterwards. They totally denied that their Rune Priests actually were psykers at all and claimed that their power came from Fenris as opposed to the warp, which was totally wrong, though their primitive belief structure caused their Underverse to manifest in a way that was completely different to the Chaos of the Warp and allowed them to distinguish between them. Of course, seeing as they use similar psychic apparatus as other librarians such hoods and foci... they might have totally known and were just being jerks. It is made clear by Malcador that Big E made a decision to tolerate their beliefs, seeing it was important for his "executioners" to be able to deal with other warp-users. In fact, two of their Librarians, Aun Helwintr and Ohthere Wyrdmake, were vocal opponents of employing Librarians at the council and yet directly participated in the Burning of Prospero.
- Prospero Burns actually nuances this, explaining that they were okay about the Librarius as envisioned but wanted Magnus censured (basically they drew a line between acceptable psyker practices and sorcery) but they failed to get their point across, leaving an opening for Mortarion to get the whole thing shut down.
- Malcador the Sigillite gave permission to the Librarians among the Knights-Errant to begin using their psychic powers after the edict, even supplying them with psychic hoods. The Adeptus Custodes made it clear that they did not recognize Malcador's authority to do so, but they were in a bit of a logical paradox over the matter: either enforce the Emperor's will by stopping Malcador from continuing his work, or defy the Emperor's will by stopping his appointed Regent from continuing his work. They settled for threatening to kill any Librarian using his powers in their presence so the Knights-Errant just did it out of their presence.
So by the Siege of Terra all but two of the loyalist Legions definitely had active Librarians, and one of the other two probably did.
Traitor Response to Nikaea
- Lorgar and his Word Bearers were already planning evil, so they just ignored the prohibition.
- Surprisingly, most of the Thousand Sons did follow the decree and murderfucked filthy Xenos the old fashioned way instead... for a while. Magnus The Red himself on the other hand decided being a ninja space wizard was too cool, and ignored the decree. If he hadn't, maybe he would've been all right.
- The World Eaters ignored the decree as well. They still had official Librarians (not Sorcerers, actual honest to goodness Librarians) during the Heresy; however, the World Eaters hated their librarians from the moment Angron introduced the Butcher's Nails and frequently didn't even tolerate them being around. This was nothing specifically personal, it was just that psykers made the Butchers' Nails a bit...twitchy and could not themselves be implanted with the Butcher's Nails without catastrophically losing control of their powers - which is not a good thing for brain implants that are already turning you into a psychotic, rage-filled murder machine. Between this and a superstitious dislike of anything "unnatural" (but only the terminally stupid would actually mention this paradox to Angron, who is unnatural in so many ways, above all unnaturally angry) there just weren't enough Librarians left to bother suppressing and the World Eaters mostly ignored them, allowing them to tag along in battle as they saw fit. They weren't represented at the council for reasons that should be obvious: Angron just didn't give a fuck because the decision didn't involve RAGE. By the time of the Shadow Crusade the remaining 19 Librarians weren't even considered full members of the Legion anymore due to their lack of the Nails. In a final showdown though, they did try to kill Lorgar before he could transform Angron into a demon prince. Needless to say, that didn't work, and Angron's first act upon attaining daemonhood was to kill them all.
- Mortarion was actually just about the biggest psyker hater in the universe, was one of the decree's strongest supporters, and was even informed by Malcador that the Emperor was planning to get rid of the Librarius about 70 years before the council even took place, giving him plenty of time to prepare for it. Hilariously, this backfired badly on him when he eventually realised that getting the Edict pushed through only weakened the loyalist's psykers; his allies were (mostly) letting their psykers run rampant with a whole new set of freakshow abilities. Things got so bad for him that he tried to get the librarian supporting Jaghatai Khan on his side. Jaghatai countered by attempting to carve the hypocrite out of Mortarion's pustuled arse. He then pillaged a Thousand Sons' library world and actually summoned a demon whose sole purpose was to tempt him into using sorcery even more. It worked, and Mortarion then yo-yoed between studying sorcery whilst pillaging every ritual device he could get his hands on, and smashing the various trinkets he acquired. It peaked with Molech before Mortarion tried to get back to first principles. Ironically due to his lack of people skills and his poor Psy-dar he never knew his second-in-command was a dirty warp burglar. First rule of management, get to know your people. As final nail in his coffin, he became a powerful Psyker himself when Granddaddy Nurgle adopted him.
- The Alpha Legion also ignored the edict like they did almost everything else. Officially, they disbanded the Librarius; unofficially, Alpharius and Omegon kept their Librarians around for special operations.
- Konrad Curze supposedly supported the use of Librarians, but his legion didn't seem too fond of psykers. The Night Lords nominally accepted the Edict, and left the decision to individual company commanders what to do with their Librarians. Most happily went along with the Edict (even going so far as to exile their Chief Librarian, who ironically was recruited into the Knight-Errants afterwards), but they occasionally dabbled in sorcery throughout the Heresy.
- The Sons of Horus went along with the decree initially, but when they got their Heresy on, they learned sorcery quickly from the Word Bearers (through the Lodges) and were quick to use it openly and without restraint after Istvaan.
- Perturabo was against using Librarians, in fact it was noted as one of the only things that brought him and Rogal Dorn together. If it might have helped the Iron Warriors build or tear down fortresses any quicker then they might have joined the "fuck Nikaea bandwagon" with gusto, but there are no recorded Iron Warriors Librarians after turning traitor either. Perturabo was pissed (as usual) that Big E used the coliseum he had been allowed to build for competitive events as a courtroom.
- Fulgrim was a proponent of the Librarius despite seeing psychic abilities as a mutation and therefore a flaw but presumably had the Emperor's Children go along with the Edict to demonstrate their perfect loyalty, since no EC Librarian has ever appeared in the fluff.
In M41 the Edict technically still stands (no one can overrule the Emperor and he never officially revoked his Edict) but it stopped being enforced after the Heresy and the marines could please themselves. Also of interest is that the Emperor violated the Edict himself by ordering the founding of the Grey Knights, who are even bigger violators of it than the Thousand Sons.
Councils of Nikaea IRL
The First Council of Nikaia (Latinly spelled Nicaea, or Nicea) was a meeting convened by Emperor Constantine in the year 325 in Nicaea not far from Byzantium. This was to sort out some controversies in Christian doctrine: in particular related to the divinity of Jesus (or lack thereof), a couple of schisms, and miscellaneous organizational details like how to deal with heretics. Slavery was also a big issue. One of the outcomes of the meeting was the Nicene Creed, which was meant to be a distillation of the most fundamental tenets of Christianity: basically, that God, Jesus, and the Holy Spirit all exist and are divine, that Jesus was sent to be crucified for humanity's sins, and that the living and dead will be judged before being resurrected into the kingdom of Heaven. The original version of the Creed includes a line saying that heretics who deny that Jesus was divine (or say that he was not always divine) are "condemned by the holy catholic and apostolic Church," but that got removed at Theodosius' First Council of Constantinople in 381; other words have been added in, for instance about the Virgin Mary, such that our "Nicene Creed" is more accurately a Theodosian Creed. Most Christian denominations adhere to the Nicene Creed to this day, the late-antique dissidents mainly having recoalesced around Islam. Although there are some Christians (such as the Jehovah's Witnesses, and the Unitarians) who have lately abandoned this.
There was also a Second Council of Nicaea in the year 787, which was convened by the Empress Irene in response to the iconoclasm (the destruction of religious icons and artwork) promulgated by her predecessors. See, the previous three Emperors had said that erecting images of Jesus and saints and so on was heresy, because one of the Commandments forbade the creation of graven images for the purpose of worship, and because paintings and sculptures could not be divine and therefore could not properly convey the holy nature of their subjects. Their opponents (the iconophiles) retorted that venerating icons of actual saints was not at all the same thing as worshiping idols to false gods, that destroying art and church property and torturing the monks who made it was hardly an appropriate response, and that the Emperors didn't seem to have a problem with people erecting images of them. In the end, the Second Council ruled that it was actually the iconoclasts who were the heretics, and that making icons and venerating them was totally okay so long as it didn't cross the line into worship of the icon, rather than the faith that the icon represented. This one is more controversial than the First Council; many Protestant denominations say that the distinction between veneration and idolatry is just playing word games. Some actually state quite bluntly that it's the Catholics who were the real heretics all along.
Since the God-Emperor of Mankind was known to be active in the early history of humanity, he probably took some amusement at the idea that he, a man who had very publicly made a point to Lorgar that he was not to be worshipped, was handing down the ban on psykers on a world named for a city in which men had decided that Jesus Christ (who may have been the Emperor Himself) was divine and that it was okay to make religious icons!
Historically, the Nicene Creed considerably shaped the world of Christianity, from the relationship of the priesthood with the Kings & Lords, to the Trinity, to ownership of humans. As a result of Trinitarian victory, it was stated that Kings no longer counted as Priests (unlike in most paganisms) nor were they of superhuman or godlike nature (unlike the Indian Raj, for instance), though the Kings of converted heathen lands often clung to the importance of divine or supernatural origins of the Royal lineage (such as Odinic origins, or the Merovingian Quinotaure) It also made the dubious claim that Jesus counts as the only priest in Christianity; the Catholic Bishops also fought against slavery inside their diocese causing it to be replaced by a dichotomy of serfdom (with the tie being formed with the land instead of with a human owner) and freemen (Churls) already present in the Pagan European substrata. Such secular concerns were arguably more important to the anti-Trinitarians participating to the Council, mimicking in a way the great economic importance of psykers in the Imperium. Ironically, these aspects of the Nicene Creed are quite in line with their 40K subversion.