The Council of Nikaea was the great conclave summoned by the Emperor to judge whether the use of psychic power had any place in His new order. Convened on the world of Nikaea before an assembly of primarchs, Legion commanders, and witnesses, it was less a debate than a trial, for the Emperor had already grown troubled by the perils of the warp.
At its heart stood a clash between two of His sons. Magnus the Red of the Thousand Sons defended the sorcerous arts as humanity's birthright and greatest hope, while others warned that such powers were a doorway through which damnation would inevitably come. Prophets, astropaths, and Librarians gave testimony, but the verdict was foreordained: the Emperor forbade the practice of sorcery and ordered the Legions to disband their psychic cadres. This ruling became the Edict of Nikaea.
The council's tragedy lay in its aftermath. Magnus, convinced he knew better, continued his studies in secret and would breach the Emperor's wards in a doomed attempt to warn Him of Horus's treachery — an act that damned his Legion utterly. Nikaea thus stands as one of the war's great hinges, a well-meant judgement that helped drive a loyal son into ruin.