Council of Nikaea

You can tell this dude in front is a Librarian (even though he's red, instead of blue!), cause of the HUGE MOTHERFUCKING BOOK on his head. Also THIN YOUR PAINTS

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 over (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 them 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. Thing is... 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 if they could ever have convinced 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.

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.

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) maintained that the Thousand Sons and - by reflection - the Librarians in general were in danger of becoming Sorcerers and had to be stopped.

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. 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. To which Rogal Dorn (who at that point was the only Primarch who had absolutely adhered to the result of Nikaea) names Russ a hypocrite because of it.

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; that guns and bullets could only do so much against an unnatural enemy against whom blades and symbols fared much better. While Russ was still proud of what he did at Prospero 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, which could have helped the loyalists win the war, 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 tactical sense, and with hindsight it forced Magnus to turn traitor anyway.

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 their 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 all 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. Though Malcador points out that the Emperor always made exceptions for Russ because his purpose was singular -for fighting other Primarchs explicitly pointing to those they do not name, then Horus and the Traitors- and that the Emperor trusted him to carry out that duty where no-one else could. This coupled with the fact that Mortarion had already been told that the Emperor planned Nikaea about 70 years before it happened, heavily implies that the intention of 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 Astropaths and those in Russ' Legion) until the Webway Project was completed. All the while playing into the Chaos Gods plans for the war that was coming. In part due to these contradictions and the special treatment he recieved, 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 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 (following Prospero, squads of Space Wolves were attached to all of the Legions to ensure their compliance with the edict anyway) This most likely boils down to whether the Emperor knew who was going to betray him and when. It has already been established that the Emperor's foresight was not infallible and the fate of the Primarchs was malleable; The Emperor's Webway project relied heavily upon Magnus operating the Golden Throne, so if there was any need to single him 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.

Loyalist Response to Nikaea

Toasty.

• 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 Nikea 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 so 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. 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, soooooo...
• 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. With the Primarch explaining 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.
• Vulkan also tried the Sanguinius approach. However, there were barely any Salamanders left after Isstvan, though 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 Nikea. 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 Nikea at the time the edict was made and could therefore plead ignorance. His "stormseer" who spoke for the Scars at Nikea 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 allowing them to distinguish between them both. Of course, seeing as they use similar psychic apparatus as other librarians such hoods and foci...they totally knew and they might just have been 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 tacit permission to the Librarians among the Knights-Errant to begin using their psychic powers, even supplying them with psychic hoods. The Adeptus Custodes made it clear that they did not recognize Malcador's authority to do so, but since the Emperor Himself had appointed Malcador as Regent of Terra, they were in a bit of a logical paradox: either enforce the Emperor's will by taking out Malcador, or defy the Emperor's will by taking out Malcador. 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.
• Magnus The Red 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, it's not even sure they were ever aware of its existence. 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 while being unnatural in so many ways, is 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 loyalists 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.
• The Night Lords nominally accepted the edict, and the Night Haunter mostly 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), as they weren't too fond of psykers anyway, 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.
• It's not really known what the Iron Warriors thought. But if it helped them build or tear down fortresses any quicker they would have joined the "fuck Nikaea bandwagon" with gusto. Perturabo was pissed (as usual) that Big E used the coliseum he had been allowed to built for competitive events as a courtroom.
• We likewise do not know how the Emperor's Children reacted. Presumably they went along with it to demonstrate their perfect loyalty, but then no EC Librarian has ever appeared in the fluff. However in Fulgrim's omnibus they point out that that they saw psychic abilities as a mutation and therefore a flaw. And because they were little bitches, they swallowed the Emperor's cock that was his edict when the time came. Later on they used sorcery to a large degree.

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.