Throughout the Great Crusade, the psychically-gifted Astartes of the Thousand Sons proved useful innumerable times as the XVth Legion fought bravely in multiple campaigns, leaving few in the Imperium in doubt of their achievements. However, there were those Primarchs such as Leman Russ of the Space Wolves, Mortarion of the Death Guard and Corvus Corax of the Raven Guard who could never bring themselves to fully trust Magnus the Red and his Thousand Sons because of their own strong feelings against the use of sorcery and psykers in general. Magnus saw the potential in exploiting the powers of psychically-talented Astartes and was instrumental in the development of the Space Marines' corps of Librarians. The Librarians quickly proved to be loyal and effective warriors and soon acceptance by