Changes

Magnus was unquestionably the most profoundly mutated of the Emperor's Primarchs, both physically and psychically, and the Legion imprinted with his [[geneseed]] reflected that , with a high percentage of Thousand Sons manifesting some level of psychic ability.{{Fn|63}} Unfortunately , the legacy of Magnus also brought with it a curse: , a debilitating flaw in the geneseed known to the Thousand Sons as the Flesh Change (also Flesh change, or Flesh-change), a degeneration—sometimes degeneration — sometimes sudden and extreme—into extreme — into [[Chaos Spawn|mindless, mutated, monstrosities]]. The Flesh Change was not detected during the Legion's inception and early years on Terra and, indeed, did not manifest within the Thousand Sons until a little over 10 years into the Great Crusade, after a seeming mass-"awakening" of apparently dormant psychic abilities in the majority of the Legion.{{Fn|38i}} The Emperor, [[Malcador]] The [[Sigillite]], and the [[Selenar]] gene-cult all expended much labour to isolate the flaw in the Thousand Sons' geneseed, but unfortunately Malcador and the Emperor concluded it was beyond their ability to cure, and it was hoped that reuniting the Legion with its Primarch would eliminate the flaw.{{Fn|63}}

The exact cause of the Flesh Change, and what triggered its onset, was not known. [[Malcador The Sigilite ]] the [[Sigillite]] initially believed it was due to an error in the XVth's "gene encoding", a belief he clung to for many years before he and the Emperor ultimately realised they could not cure the Thousand Sons.{{Fn|63}} There may be a link with sustained, heavy, use of psychic powers, as the Thousand Sons fell en mass to the Flesh Change during the burning of Prospero, when the Thousand Sons desperately unleashed ever more powerful "sorceries" at the Space Wolves and Custodes of the Censure Host to try and defeat them.{{Fn|3p}} Another victim who fell under similar circumstances was [[Revuel Arvida]], who continuously used his powers to assist the White Scars, during their campaigns against the [[Death Guard]] and other Traitor forces.{{Fn|64}} However, this may simply be correlation and not causation, as Thousand Sons legionaries who completely lacked psychic ability were just as susceptible to the Flesh Change.{{Fn|3q}}