Warmaster Phthisis wrote: The leader of an obscure warband of Nurgle cult marines exemplifies a unique aspect of Nurgle's gestalt entity that makes him equally dangerous to the chaos gods ...
This right here is actually a bigger problem than any of the fluff relating to the Eldar. How on earth could this Nurgle champion be "dangerous" the Chaos Gods? Chaos Gods aren't gonna catch a head cold from him, no matter how toxic the virus is. The gods are beings of
raw warp energy and emotion. If this Nurgle leader becomes strong enough, the plague and despair he sows will make Nurgle MUCH STRONGER, not weaker. Maybe the other gods will be weakened a little as Nurgle supplants them, but all that war, strife, desperation, and scheming the Nurgle champion leaves in his wake is still gonna keep them fed. It may just be hyperbole, but take this part out. A Nurgle champion so powerful and uber important that he threatens the Chaos Gods (including the one he serves) is Mary Sue-ish.
As far as the rest of it goes, with the Farseer foreseeing that this champion will destabilize the galaxy in the exact right way that the Eldar can restart their empire, so he starts subtly helping the champion and winds up found out and hunted by other Eldar, I can buy that. It could be interesting. The thing is, it's highly likely that this Farseer's vision was all a Chaos trick, that he's gone mad with despair, and that he's being corrupted the whole time and just doesn't realize it. That's certainly how the other Eldar are gonna see it. I'm inclined to agree with them. If you want a dang good story for your campaign, keep it ambiguous whether he's right or not. Make a good case for both sides. Let the other players make choices based on what they believe about this Farseer's vision and split into factions that way.