The Book of Magnus, also referred to as the Book of the Thousand Sons, is a massive tome containing the collected knowledge of sorcery as dictated by Magnus the Red, the Primarch of the Thousand Sons Legion, during the Great Crusade in the late 30th and early 31st Millennia. It is one of the most complete treatises on psykers, witchcraft and sorcery in the galaxy, a compilation of all the knowledge and experiments gathered by Magnus during the conquests the Thousand Sons made for the Imperium during the Great Crusade. This esoteric tome was copied down from the Primarch's own words by the Remembrancer Mahavastu Kallimakus, a meticulous recorder of data and a fastidious observer of details. He had recorded much about the earliest days of the Great Crusade as he had been one of the first Rem