Edict of Nikaea

The Edict of Nikaea was issued during the Council of Nikaea in the early years of the Great Crusade. Following debates about the dangers of psychic powers, the Emperor decreed that the Librarius divisions of the Space Marine Legions be disbanded and that no Astartes should practice psychic arts. The Edict was a direct response to fears that the Warp could corrupt the Legions, particularly after the Thousand Sons' continued use of sorcery.

This document remains one of the most consequential decrees in Imperial history, as its enforcement — and the resentment it caused — played a significant role in the events leading to the Horus Heresy.