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The planet was in its infancy at the time of the Council and was terraformed by the Adeptus Mechanicus at the Emperor's request. Ultimately the use of psykers was allowed but restricted, while sorcery was officially banned. Magnus the Red and his legion The Thousand Sons would continue to study such powers in secret.
There was an increasing concern as the Great Crusade progressed forth considering the use of Sorcery. More and more often the Imperial Army and Space Marines would make planet fall only to find the populace thralls to mysterious powers. These people were essentially slave cults who would resist the forces of the Emperor with sorcerous powers granted them by daemonic beings from across the Warp. Powers that were very akin to those used by the Thousand Sons of Magnus.
Mortarion who knew by his own personal experience the dangers of the warp, and Leman Russ for whom any battle fought through sleight of hand and clever deceit was by definition dishonorable, highly criticized the Thousand Sons. The schism grew so great that it threatened the very stability of the fledgling Empire and so the Emperor himself decreed a council to resolve the issue once and for all.
Both sides of the debate were gathered to the planet Nikaea to give their views, with the Emperor as an arbiter, enthroned above the dais in an ancient amphitheater that seated tens of thousands.
On one side: Witch hunters presented their case reciting a litany of human suffering inflicted upon the Emperor's own subjects by sorcerers enslaved by Chaos, of gibbering mutants who had lost all humanity, and of cults and power hungry men who turned their psychic gifts to dark purposes.
On the other: Magnus himself. His very presence frightened many, but he began to speak with great charisma. His argument was that no knowledge was tainted of itself, and no pursuit of knowledge was ever wrong so long as the seeker of that truth was master of what he learned. He spoke with finality that the Thousand Sons had mastered their knowledge and that there was no ways too labyrinthine for them to know. 
Magnus had spoken passionately with great power and the Council became even more divided. While effective the Witch Hunters could not 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 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 Epistolary who spoke.
A psyker, he proposed, was like an athlete, a gifted individual whose native talent must be carefully nurtured. Psykers were not evil in themselves. Sorcery however was a knowledge that had to be sought for, even bargained for with evil powers. No one could be sure who benefited in the deal.
They proposed the education of all psykers and their priority would be to serve Mankind. This should become an immediate Imperial priority. Conducting sorcery would forever be outlawed as an unforgivable offense against Mankind and the worst kind of heresy.
The end result was a compromise that offered both factions something. It was as if this was what the Emperor was waiting for, and he ruled it as law immediately without allowing any rebuttal from either side. The Edicts of Nikaea stand to this millennium as the Imperial policy regarding human psychic mutation. Magnus attempted to storm out of the hall in protest, but was barred by the Emperor himself. The confrontation between father and son is recorded in the Grimoire Hereticus.
The Emperor ordered Magnus 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 it was said his face was brittle enough to crack. But in the end he bent his will to the Emperor and agreed to obey. 
Following the Burning of Prospero, a shard of Magnus representing his feelings of betrayal came to rest on Nikaea. During the hunts for the Shards of Magnus, the Imperial party led by Dio Promus, Yasu Nagasena, and Bodvar Bjarki confronted Ahriman and his Thousand Sons on the world. A furious battle erupted, and the shard-possessed body of Lemuel Gaumon summoned a miniaturized Crystal Labyrinth. Ultimately thanks to the efforts of Ahriman, the Shards of Magnus were able to reunify and returned to the Planet of the Sorcerers with the Thousand Sons.
At the First Council of Nicaea, in 325 AD Constantine the Great - the first openly Christian Emperor of Rome - invited 1800 bishops to meet together for the first to time establish Christianity as a single consistent faith. One of the issues on the meeting's agenda was the "Arian controversy."
On one side: St. Alexander of Alexandria and his followers debated that Jesus was son of God and was the same substance of the father and is co-eternal with him. On the other: Arius himself. Arius and his followers believed that Jesus was born/created by God, but was essentially lower than God as just a creation.
The Emperor presided over the debate, which was won by Alexander.
There are several strong parallels between this real-life event and the Council of Nikaea; in particular, there is a rumour that Saint Nicholas (who is tied to the myth of Santa Claus) slapped Arius about the face. Saint Nicholas is prominently tied to Norse mythology, which may account for Leman Russ playing an analogous role to Nicholas at the Council.
- 1: The Horus Heresy Book One - Betrayal, pg. 10
- 2: A Thousand Sons (Novel) by Graham McNeill, [Needs Citation]
- 3: Index Astartes III, [Needs Citation]
- 4: Prospero Burns (Novel) by Dan Abnett, [Needs Citation]
- 5: The Crimson King (Novel) Chapter 15