Emperor of Mankind - Council of Nikaea: Additionally, there was an increasing concern as the Great Crusade progressed about the use of psychic sorcery by agents and warriors of the Imperium. The Emperor was the most powerful Human psyker to have ever lived, but He was deeply ambivalent about the growing spread of the mutant psyker genes through more and more of the Human population.He rightly believed that most of Mankind was not yet evolved enough either physically or spiritually to truly control the great power of the Warp or avoid the corruptions and temptations offered by its more malevolent denizens. More and more often during the progress of the Imperial conquest of the galaxy, the Imperial Army and Space Marines would make planetfall only to find that the populace were in thrall to mysterious powers and unnatural mystics called "sorcerers."These people were essentially members of Chaos Cults who would resist the forces of the Emperor with sorcerous psychic powers granted them by Daemonic entities from the Warp. These psychic powers were also very akin to those used by the Thousand Sons Legion of Primarch Magnus the Red.The Thousand Sons had come under intense criticism for their use of sorcery by Primarch Mortarion of the Death Guard Legion, who knew by his own personal experience with sorcerers on his homeworld of Barbarus the dangers to be found in anything spawned from the Warp, and Leman Russ of the Space Wolves Legion, for whom any battle fought through sleight of hand, clever deceit or any trick other than straight physical combat was by definition dishonorable.Russ found the Thousand Sons' use of sorcery distasteful in the extreme. It was Russ who fought the hardest for the Imperium to ban the use of psychic powers after his own experiences during several campaigns of the Great Crusade where his Space Wolves had fought beside the Thousand Sons.The schism grew so great that it threatened the very stability of the fledgling Imperium and so the Emperor Himself called for an Imperial conclave to resolve the issue once and for all.Both sides of the debate over the use of psychic abilities arrived at the world of Nikaea determined to present their views, with the Emperor as the arbiter, enthroned above the dais in an ancient amphitheater that seated tens of thousands where the conclave was held.On one side of the question were the Witch Hunters like the Sisters of Silence who presented their case by reciting a litany of Human suffering inflicted upon the Emperor's own subjects by sorcerers enslaved by what would eventually later be recognised during the Horus Heresy as Chaos, of gibbering mutants who had lost their Humanity, and of cults and power-hungry men and women who turned their psychic gifts to dark purposes. All present were also aware of the terrible damage that had been done by uncontrolled and Daemon-possessed psykers during the early days of the Age of Strife.On the other side was a powerful advocate for the continued use of sorcery, Primarch Magnus the Red. His very presence frightened many, but he began to speak with the great charisma that only a primarch could wield. His argument was that no knowledge was tainted in and of itself, and no pursuit of knowledge was ever wrong so long as the seeker of that truth was the master of what he learned rather than its pawn.He spoke with finality that his Thousand Sons Astartes had mastered their knowledge of sorcery and that there was no knowledge too labyrinthine for them to grasp or that they could not master to serve Humanity rather than enslaving it. Magnus called on the Emperor not to ban the use of psychic abilities, but to contribute to further research into their usage so that they might be harnessed more fully for the betterment of Humanity and the Imperium.Magnus had spoken passionately with great power and the participants at the Council of Nikaea became even more divided. While they had strong arguments in their favour to justify their anti-psyker position, the Witch Hunters could not effectively match Magnus' persuasiveness. The tension could easily have been cut with a knife when a group of Space Marine Librarians approached the dais.The Emperor acknowledged them with a nod, and all present fell silent. Among the group were some of the greatest Librarians of the Space Marine Legions. They formed a semi-circle around the dais to indicate that they spoke as one voice, but it was a young Librarian Epistolary who spoke for the group.A psyker, he proposed, was like an athlete, a gifted individual whose native talent must be carefully nurtured. Psykers were not innately evil in themselves, but like any tool, could be used for either good or evil purposes. Sorcery, however, was the knowledge of how to wield psychic powers that had to be sought for, even bargained for with the foul entities of the Warp. No one could be truly sure who or what had benefited in the deal.The Librarians proposed that all psykers be strictly educated by the Imperium with the express purpose of using their abilities to serve Mankind. This should become an immediate Imperial priority. The practice of sorcery would forever be outlawed as an unforgivable offense against Humanity and the worst kind of heresy.The Council of Nikaea was also the trial of Magnus the Red -- for he was accused of sorcery and of introducing sorcerous practices to the Space Marine Legions through his lead role in the institution of the corps of Librarians. As the evidence of Magnus' continued practice of sorcery became apparent, the Emperor barely contained His wrath as He pronounced judgement on the primarch of the Thousand Sons.The Emperor had entrusted His son years before to obey His bidding and foreswear the use of such occult practices because of the dangers inherent to the Warp. He had entrusted only Magnus with the true secrets of the Warp to which only they remained privy, but now it appeared that His son had disobeyed His edicts and at the very least dabbled in the occult and the forbidden black arts of psychic sorcery. The confrontation between father and son is recorded in the Grimoire Hereticus.The Emperor's judgement at the Council of Nikaea proved severe, largely as a result of His anger at Magnus. The Emperor rejected the Librarians' proposed compromise outright. With the exceptions of Navigators and astropaths who were properly trained, controlled and sanctioned by the Imperium and were necessary to its continued existence, the Space Marine Legions were no longer to employ psykers within their ranks.He commanded that the primarchs were to close their Legions' Librarius departments forthwith and not to indulge the undoubted psychic talents of those Asartes who possessed the gift. All existing Space Marine Librarians were likewise forbidden to make use of their abilities and were to return to duty as line warriors.The Council's rulings also created a new position amongst the Space Marine Legions, the Space Marine Chaplain, to uphold the Imperial Truth and help maintain the purity of an Astartes Legion's dedication and fidelity to the Emperor's commands.The Emperor ordered Magnus once more to cease the practice of sorcery and incantation, and the pursuit of all knowledge related to magic. Magnus, of course, did not like the idea, and he remained bitterly opposed to the decision made at Nikaea. But in the end, he bent his will to his father the Emperor and agreed to obey, though the machinations of the Ruinous Powers would ultimately lead to a far darker fate for Magnus the Red and his Thousand Sons.The Edicts of Nikaea stood largely untouched for the next 10,000 standard years as the primary Imperial policy regarding Human psychic mutation. Only the edict against the use of Librarians within the ranks of the Space Marines and later the Astra Militarum would be reversed as a result of the Horus Heresy, as that terrible civil war made clear to the rulers of the Imperium after the conflict that Astartes and other Sanctioned Psykers were essential to combat the power of the forces of Chaos.