The Ravonicum Rex is an ancient and possibly apocryphal text whose origins are unknown to the Imperium. According to this source, there were some amongst the Gathering of the Thousands that sought a compromise over the issue on the use of psykers and the arts of sorcery that were debated in the Council of Nikaea. These individuals attempted to see the merits of each side in the intractable debate over the use of psychic abilities.
A copy of this book was kept within the deepest recesses of the Librarium Sanatorium onboard the Blood Ravens ship Omnis Arcanum. The relevant sections of this text became a hotly debated piece of lore amongst that Chapter of the Adeptus Astartes despite its mysterious origins. The Great Father Azariah Vidya alluded to in his Pax Psykana that a complete copy of the Ravonicum exists but vanished after an Eldar Harlequin raid on the planet Quarab.