Thousand Sons - The Great Crusade: Approximately 5 standard years after the Thousand Sons began the Great Crusade to reclaim and reunify the Human-settled galaxy, the Thousand Sons' began to display powerful psychic abilities, which was at first a welcome development for its Astartes as they sought to further emulate the Emperor.This development was reluctantly tolerated by the Emperor, who was greatly wary of psychic abilities wielded by anyone other than Himself or Malcador the Sigillite, because the Thousand Sons' abilities proved to be a powerful weapon for the Imperium during the Great Crusade.Throughout that long campaign, the Thousand Sons made extensive use of sorcerers, and their Warp-derived powers would leave whole populations in thrall to their will rather than carrying out a planetary conquest through a costly full frontal assault like the other Space Marine Legions. This tactic earned the ire of Leman Russ of the Space Wolves Legion who saw anything less then a frontal assault as dishonourable and cowardly.But soon, the Legion's joy at their psychic gifts turned to revulsion and horror as a wave of ghastly, degenerative mutations began to affect large portions of the battle-brothers of the XV Legion. These mutagenic changes began to be called the "Flesh-Change" by the Legionaries, and were much feared, as the spiral of degenerative mutation ultimately reduced a proud Thousand Sons Astartes into a mindless mutant abomination that would later be recognised by the Imperium as a gibbering Chaos Spawn.The majority of those afflicted by the Flesh-Change were put into stasis by the Legion in the hope that someday in the future a cure would be able to be found to 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 the Flesh-Change.Fortunately, for the XVth Legion the Emperor's Great Crusade finally arrived at the isolated world of Prospero and the Thousand Sons were reunited with their Primarch Magnus. The Flesh-Change had become a pandemic at this point, but the entire Legion was transported to the newly discovered world to meet their gene-father. After the initial reunion with one of His lost sons and the celebrations that followed, the Emperor and the vast majority of the Great Crusade's large fleet departed Prospero, leaving the Thousand Sons behind.By then the rampant mutations within the Legion had spontaneously gone out of control after the departure of the Emperor. The Thousand Sons Primarch Magnus intervened to save his genetic children, and through mysterious means was able to save those that had been the least affected by the rampant mutations. In the end, the XVth Legion only numbered a thousand battle-brothers that had managed to be saved.The Thousand Sons gene-seed had been stabilised and the recruitment of potential initiates from Prospero began to replenish the Legion's severely depleted ranks. The XVth Legion was eventually able to rebuild its numbers (though the Thousand Sons Legion would never become as large as its fellow Legions) and return to the Great Crusade, led by their Primarch Magnus. Using their psychic gifts, the resurgent Thousand Sons Legion brought a number of worlds into Imperial Compliance, bringing them into the Imperial fold.Their primary methods of victory included the use of diplomatic guile and trickery, a modus operandi that was not questioned by the rest of the Imperium. But these methods grew less effective as the XVth Legion's expeditionary fleet entered more dangerous regions of unknown space where extensive hostile forces were encountered. Many of these new enemies of Mankind employed psychic powers similar to those that the Thousand Sons wielded.The XV Legion used their own potent psychic gifts and sorcerous abilities to bring these enemies of Mankind into Compliance or to exterminate them altogether. Over the following solar decades some of the primarchs voiced their dissent that these so-called "psyker" Legionaries were allowed to exist and be a part of the Emperor's righteous Great Crusade.