During the early days of the Great Crusade there was a great conclave called by the Emperor of Mankind known as the Council of Nikaea. This conclave was intended to determine whether or not the use of psychic abilities represented a boon or a grave danger to Mankind and the newborn Imperium of Man. Ultimately the existence of psykers in the Imperium was allowed but tightly restricted under centralised Imperial control, while the potent and unrestricted use of psychic abilities that was defined as sorcery was officially banned. This Decree Absolute was a command of the Emperor Himself, a warning about the dark potential for corruption that was implicit with the use of the powers offered by the Warp. This was a command that the Primarch Sanguinius echoes, to forbid the use of preternatural powers within the IX Legion. A command the Blood Angels accepted without question.
The Emperor did not make his decision lightly, but following the actions of Magnus the Red and his Thousand Sons Legion, there was little choice. There had always been those who looked unkindly upon the use of the powers of the mind and saw only the hazards that they encompassed. The great psyker-Primarch Magnus had brought all that opposition to a head with his reckless exploration into the deeper, darker places of the Warp, drawing his father’s great displeasure and this draconian response. The acolytes of the Word Bearers had been sent to many expeditionary fleets, placed in several Legions in the months that unfolded after the passing of the Edicts of Nikaea. The suspension of psychic warfare and the abolition of the Librarius contingent had been dealt with differently in each Legion that maintained one, each according to their individual traditions and methods. In a service offered by the Primarch Lorgar to his brethren, the master of the Word Bearers Legion had sent his most pious and vigilant apostles to help with the reintegration of those gifted with psyker powers back into the rank and file of the various Space Marine cohorts. This effort had given rise to the newly established Chaplain corps. No help from the Word Bearers had been requested or required by the Blood Angels, however. The black-armoured Wardens, their roles already embedded in the Legion proper, had taken on the task of policing the reformation of the IX Legion according to the Edicts of Nikaea. The Blood Angels' Wardens were not the same as Lorgar's Chaplains, though they ultimately would come to serve the same function within the Legions and the later Second Founding Chapters of Space Marines.
The Wardens were the watchmen of the Blood Angels. In some ways they served as mentors for the younger Astartes, battlefield instructors and learned veterans who shared knowledge with the rest of their kindred; but they were also charged with sustaining coherence throughout the tens of thousands of warriors that filled the ranks of the IX Legion. That could mean anything from offering suggestions to a Captain on a point of combat doctrine, to leading a ceremony of remembrance to the fallen. They served as lore-keepers, counsellors, and teachers. In the deep past, men who had served in similar roles in other militaries had been known as diaconus, zampol, chaplain or a dozen other names -- some political, some religious, some secular. The Blood Angels' Wardens existed outside the Legion's chain of command but still within its ranks, maintaining that most Imperial of ideals throughout the Legion -- unity.
The primary responsibility of High Warden Berus and the other Blood Angels Wardens was to ensure that the Emperor's Decree Absolute was strictly enforced, monitoring those Battle-Brothers who were former members of the Librarius and had been reintegrated back into the IX Legion's line companies. During his tenure with the Legion during the Great Crusade and the Heresy, Berus continued to provide spiritual counseling and guidance to both his fellow Battle-Brothers as well as those few that were psychically gifted. The Wardens always stood ready to renew a Battle-Brother's devotion when he faltered, and to remind him of his duty and his vows when hope seemed lost.
- Fear to Tread (Novel) by James Swallow