Doodling some ECs and I came to realise that by the time Emperor's Children fall to chaos after Laeran campaign, it seems all of original 200 Astartes of Third who survived Unification Wars and harm of Selenite plot and were given to Fulgrim, except Fabius of course, are already dead and replaced with healthy, newly recruited Astartes who do not suffer from the Blight, because they got brand new geneseed straight outta Dad's own stock *wink wink*.
Which, again, might not be very relevant fact, but at the same time I feel it's deeply meaningful to whole essence of what Emperor's Children represent and their story arc. Old died because they were flawed, except one who believes he can cheat death purely on his scientific genius (and Fabius, you crazy sonofabitch you did it) and new were created, with Fulgrim's own geneseed. You can see it as a start to corruption in a way, as old ECs were much more self sufficient and individualistic (as many of old, pre-Primarch astartes tended to be) while new ECs rely on Fulgrim in all ways imaginable. He became a poision to them with his own insecurities and nearly maniacal need of perfection.
So yeah, considering all this, and knowing how much of mythology and religious lore is intertwined in Warhammer universe, I can see why snakes are dominant motif in ECs fall off grace to Slaanesh: Adam and Eve story of serpent offering forbidden fruit to both of them, them taking it and thus God casting them off the Garden of Eden...
