Battle of Prospero - Sorcery of the Thousand Sons: After the Emperor arrived on Prospero, Magnus was reunited with his genetic progeny amongst the XVth Legion of the Space Marines.The XVth had been formally named the "Thousand Sons" by the Emperor and given their own expeditionary fleet so that they might begin to make contributions to the Great Crusade the Emperor had unleashed upon the galaxy to reunify all of Mankind beneath the aegis of the Imperium of Man.Like their gene-father, the Thousand Sons had inherited powerful psychic talents, as their gene-seed organs possessed their primarch's own psychically-potent genes. However, the Legion's gene-seed was also genetically unstable, and often produced unwanted mutations known as the "Flesh-Change" by the Legionaries.The Flesh-Change was much feared amongst the Thousand Sons, as the spiral of degenerative mutation ultimately reduced a proud Thousand Sons Astartes into a mindless mutant abomination similar to a Chaos Spawn. The majority of those Thousand Sons afflicted by the Flesh-Change were put into stasis by the Legion in the hope that someday in the future a cure would be found that could reverse the onset of these crippling mutations.The number of active Astartes within the XVth Legion soon began to dwindle to dangerously low levels as a result of the ravages of constant mutation. Some within the Imperium called upon the Emperor to declare the Legion hopelessly corrupted and remove them from the Great Crusade's active order of battle.By the time the Thousand Sons were reunited with their Primarch Magnus, they numbered only one thousand active Astartes -- literally the "thousand sons" of Magnus. Through unknown means after his restoration to his Astartes, Magnus gained the knowledge required to cure his Legion's tendency towards rampant mutation.He even restored his sons who had been afflicted with the Flesh-Change and placed in stasis, making the XVth Legion whole once again. The Legion quickly adopted Prospero as its new homeworld and many of Magnus' followers from amongst the sorcerer cults on the planet were recruited into the Legion's ranks.Despite the Emperor's earlier warnings, the primarch continued to obsess over the acquisition of the forbidden knowledge of the Warp and never ceased to practice sorcery. Magnus secretly taught the Thousand Sons the ways of the sorcerers of Prospero, and to embrace their psychic talents as a gift rather than suffer them as a curse. Within a short period of time the Thousand Sons had become a secret cabal of powerful Warp sorcerers.Throughout the Great Crusade, the psychic talents of the Thousand Sons proved useful innumerable times as the XVth Legion fought bravely in multiple campaigns, leaving few 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 and his Thousand Sons because of their own strong feelings against the use of sorcery and psykers in general.Leman Russ, the product of a feral barbarian culture on the Death World of Fenris where all psykers were feared as fell witches, condemned the Thousand Sons as warlocks who employed fell magicks.Mortarion openly accused Magnus of dabbling in foul sorcery similar to that exercised by the hated Overlords of Barbarus, and Corax twice refused to field his Legion alongside the Thousand Sons.