Magnus the Red - Great Crusade: When the XVth Legion had first been created, its Astartes were used to quell the last few pockets of isolated resistance to the Emperor's rule that remained on Terra following the Wars of Unification. Boetia was a nation that had existed since the Age of Strife and was chiefly known for holding out against the Unification for a considerable period of time until it was finally forcibly incorporated in the newborn Imperium. The ruling family, the Yeselti, attempted to stubbornly cling to power, forcing Imperial Army forces to invade their province. The Astartes of the newborn XVth Legion were sent in to finally crush any further resistance in a brilliant campaign that lasted only six weeks. Shortly after achieving this objective, the XVth Legion was formally named the Thousand Sons by the Emperor Himself and was sent out into the galaxy as part of his Great Crusade. This particular conflict holds the distinction of being the final campaign of the Unification Wars that took place during 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 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, but 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 the Primarch Leman Russ of the Space Wolves Legion who saw anything less than a frontal assault as dishonourable and cowardly. But soon, the XVth Legion's joy at their psychic gifts turned to revulsion and horror as a wave of ghastly, degenerative mutations once more manifested across a wide swathe of the Battle-Brothers of the XVth Legion. These mutagenic changes were known as the "Flesh-Change" amongst the Astartes of the Thousand Sons, and were much feared, as the spiral of degenerative mutation ultimately reduced a proud 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 could 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 at this time. The flesh-change had become a pandemic in the Legion at this point, but the entire Legion was transported to the newly discovered world to meet their gene-father. After the initial joyous 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. Unfortunately, the rampant mutations within the Legion only increased after the departure of the Emperor. It was then that Magnus intervened to save his genetic children, and through mysterious sorcerous means was able to save those Astartes that had been the least affected by the rampant mutations. In the end, only a thousand Battle-Brothers of the XV Legion had been saved. As mentioned above, Magnus then moved to rebuild the Legion, and approximately 100 standard years into the Great Crusade, the Thousand Sons were granted permission to form the 28th Expeditionary Fleet and begin bring new worlds into Imperial Compliance for the Emperor.