Julian's Rum has quite the story going, here. Apparently, it's on Wikipedia that this is a Bible theory that is wholly adopted by "White Supremacist" in their "Christian Identity" theology.
https://en.wikipedia.org/wiki/Serpent_seed
This belief is also held by some adherents of the white supremacist theology known as Christian Identity, who claim that Jews are descended from the serpent.
This is interesting because after thinking about it a bit, it reminds me of how the Abrahamic blessing works. It's skips the oldest, and goes to the second. If Cain was illegitimate, then Adam's first son would be Able, and Seth the second son.
Here's a vid I've passed around a few times as I was looking into why the cabal puts such emphasis on lineage.