I was hoping it wasn't going to turn out to be Emrakul, since I don't like eldrazi at all. But everything is hinting very strongly in that direction.
The plot seems to be that Nahiri is drawing Emrakul to Innistrad as revenge against Sorin, and Emrakul's approach is causing the madness and corruption. But it could still be something else!
Well, currently that is the explanation I lean towards. Nahiri was pissed that Sorin wasn't there when the Eldrazi Titans got freed from their prison. I.e. Nahiri blames Sorin for the near destruction of her homeplane. That is why she made the cryptoliths that lead to the drownyard, where cultists worship...something. The circle at the drownyard is basically the landing strip for Emrakul, indicating where she is supposed to land. I.e. one gigantic lure. How is it a landing strip? It is my belief that the cryptoliths are channeling ley lines to one point, making for a huge focal point of mana. Obviously that will attract Emrakul.
That the madness and corruption is due to Emrakul is hinted at in multiple cards. Take, for instance, Paranoid Parish-Blade.
Look closely at his left hand. Well, it doesn't have to be closely since it's practically screaming at you in that resolution.
Then there's obviously Grotesque Mutation.
Now what do we know that also had purplish/reddish/blueish tentacles?
Oh right.
There might be some more hints in other cards, but I haven't found them so far.
Lastly, some whispers abound that Nahiri destroyed Markov Manor because Sorin spread vampirism to Zendikar way back when.
