The Edict of Nikaea was the Emperor's binding decree, issued after the Council of Nikaea, that outlawed the practice of sorcery within the Legions and the wider Imperium. Psychic soldiers were to lay down their arts, the Librarius cadres were to be dissolved, and the study of the warp's deeper mysteries was forbidden as a peril to all mankind.
The Emperor framed the edict as protection rather than punishment. He alone understood how readily the powers of the empyrean corrupted the unwary, and He sought to shield His sons from a doom He could see approaching. Yet the decree was resented and unevenly obeyed. Some Legions embraced it; others quietly kept their psykers; and Magnus of the Thousand Sons defied it outright, certain that forbidden knowledge was the Imperium's only defence.
In the end the edict achieved the opposite of its intent. By severing the Legions from disciplined psychic teaching it left them ignorant and vulnerable when Chaos struck, and its harshest consequence — the assault on Prospero — turned a loyal Legion into exiles and eventual traitors. Like so much of the Age of Darkness, a decision made in wisdom bore only bitter fruit.