It is later revealed to Tarvitz within Galaxy in Flames that Eidolon and Fabius Bile have been covertly developing and conducting a massive and extensive genetic/biological modification and enhancement program for quite some time. The plan is to select the best and the brightest of the Emperor's Children and modify their physiques and their forms with novel enhancements developed and bred by Bile and his servants. These modifications range from enhanced strength and agility to an organ that generates a piercing sonic scream (Eidolon reveals that this ability is indeed the result of Bile's modifications and actions) and myriad other alterations/supplements, or so it is implied by the vast range of varying organs, modifications, apparatuses, etc. described in Tarvitz's rather brief assessment of the pertinent facilities. It does not appear that the Legion has been directly tainted by Slaanesh at this point or that any explicit corruption of materials has occurred; however, it is possible that the designs and developments of Bile where influenced, inspired, or even generated directly, or more probably, indirectly by the Fell Powers. Indeed, if memory serves Fulgrim actually catalysed the project to begin with, so it may well be that the psyche of Fulgrim (which most probably has been afflicted by the Fell Powers even at this stage) and it may well have been that this spurred him to enact such a project to further perfect the Legion, or loosened his moral perceptions to the extent that he was willing to do so.