Changes

At the beginning of the Crusade, they fought as tenacious and energetic expanders of the [[Imperium]], and were not considered particularly distinct from the main body of the [[Legiones Astartes]]. Five years into the Crusade , however, the warriors of the legion—much to their delight—all began to spontaneously develop [[Psyker|psychic abilities]]. This development however, was followed by a wave of horrific, unwilling degenerative mutations. This mutative process became referred to as the [[flesh-change]], and was much feared by the Thousand Sons after it first appeared during the Compliance of [[Bezant]]. Most of those afflicted were secured in [[Stasis field|stasis]] chambers in the hope that some future cure would be found for them, and the numbers number of active Thousand Sons began to dwindle as a result.{{Fn|3a}}

Over the next several decades some of the then-discovered primarchs found the notion of psychic mutants being allowed to exist as part of the Emperor's Crusade distasteful at best and downright impossible at worst and began to variously move for their censure or spread rumours and condemnations about them. Chief amongst these detractors were [[Mortarion]], [[Rogal Dorn]] , and [[Corvus Corax]]. As the years passed, more and more of the Thousand Sons devolved into mutantcymutation, while those that survived grew stronger and stronger in the use of their powers. Meanwhile, the detractors had managed to raise enough voices to empower a proposal that the Fifteenth Legion be disbanded and expunged from [[Imperialimperial]] records altogether.{{Fn|3a}}

It was shortly after this that the Great Crusade arrived at Prospero and found Magnus, primarch of the Thousand Sons. The entire legion was transported to Prospero to meet their gene-sire, and there was much rejoicing , even though the flesh-change had become a pandemic within their ranks. After the initial celebrations , the Emperor and the main body of his fleet left Prospero, but the Thousand Sons remained behind, the numbers number of them degenerating into mutantcy mutation seeming to go completely out of control within a day of the Emperor's departure. Soon, the entire legion was afflicted. It was at this time that Magnus stepped in to save his gene-sons, enacting a mysterious procedure that was able to stabilise the genetic structure of the least-affected legionaries. The procedure turned out to involve agreeing to an open -ended bargain with a Warp creature called [[Choronzon]], the Dweller in the Abyss and the Daemon of Dispersion.{{Fn|3s}} At the end of the procedure, barely a ''thousand'' of the ''Thousand'' Sons " "lived."{{Fn|3a}}.