Magnus the Red - Discovery by the Emperor: With such a potent mind within the Warp, it was not long until the Emperor of Mankind noticed Magnus' presence and directed His Great Crusade expeditionary fleets to make for Prospero. When His fleet arrived at Prospero in 840.M30 and the Emperor set foot within the grand precincts of Tizca, He and Magnus immediately embraced and conversed as if the two had known each other for many standard years.Following his rediscovery, Magnus and the Emperor engaged in solar-decades-long joint travels and study of the Immaterium, with the Emperor imparting further knowledge to His gene-son, along with multiple warnings about the dangers inherent in overindulging the gifts of the psyker due to the dark entities that existed within the Warp and sought to manipulate mortals for their own benefit.The XVth Legion, the Thousand Sons, had inherited Magnus' psychic talents, as their gene-seed had been created from samples of Magnus' own psychically-potent genes. However, the XVth Legion's gene-seed was genetically unstable, often resulting in rampant and unwanted mutations known as the "Flesh Change," Space Marine-organ rejections, and mental instability. The Legion was dramatically under-strength because of these problems and its eventual survival was questionable. It was said the Legion owed its name to the fact that only about a thousand stable Astartes remained functional within it by the time Magnus was found on Prospero by the Emperor.The Thousand Sons Legion was not allowed to join the Great Crusade at its beginning, with some in the young Imperium advocating its disbanding and the euthanising of its members; however Magnus pleaded with the Emperor to give him a chance to find a way to stop the rampant mutations after he accepted his rightful position of leadership over the Legion. The Emperor agreed to this, and after several solar decades of often desperate effort, Magnus was finally successful: the aberrant mutations decimating his Legion stopped.This effort had a visible cost, and a hidden one: Magnus lost his right eye, the skin becoming smooth where the eye had previously been. To stop the mutations, Magnus had willingly consorted with Daemonic entities from the Warp whose true nature he did not at the time understand. He was unknowingly fooled by the Chaos God Tzeentch into believing he had bested the Warp entities and that he had genuinely found the "cure" for his Legion though in fact he had only made a devil's bargain for the sorcerous power required to stop temporarily the mutations. All of this was a ruse crafted by Tzeentch to make Magnus even more arrogantly beholden to his psyker's gift to drive him to pursue more corrupting forms of sorcery that would eventually lead to his corruption by Chaos.Because of the setbacks that had befallen the Thousand Sons and the continuing suspicion of many in the Imperium towards the Thousand Sons' use of psychic abilities and the flaws in their gene-seed, the primarch and his Astartes developed an extremely close emotional bond. Approximately 100 Terran years after it had begun, the Thousand Sons Legion was finally permitted to join the great effort as the 28th Expeditionary Fleet of the Great Crusade.