The Librarius was the psychic corps of a Space Marine Legion, the body of warrior-psykers — Librarians — who turned the powers of the mind to war. Trained to summon lightning, shield their brothers, and pierce the fog of battle with foresight, they were among the most potent assets any commander could field during the Great Crusade.
Yet the Librarius existed under a shadow of suspicion. Many feared that to wield the warp was to court its masters, and the debate over such powers culminated in the Council of Nikaea, whose edict ordered the Librarius disbanded across every Legion. Some obeyed and lost a vital strength; others concealed their psykers or, like Magnus's sons, refused the ruling altogether.
When the war came, the wisdom of Nikaea looked like folly. Legions that had cast out their Librarians found themselves defenceless against the sorceries of the traitors, and in the conflict's later years the loyalists quietly rebuilt their psychic cadres out of grim necessity. The Librarius endured into the Imperium of the 41st Millennium as a permanent institution — proof that the powers the Emperor had tried to forbid could not, in the end, be done without.