If one goes on the basis of what Jordan said, there is nothing outlandish about it. He said it was an "attack of opportunity" (effectively ruling out Slayer, to my mind) and that we had enough information to figure it out by the end of Book 5. The obvious answer is Lanfear, given how often she threatened to kill Asmodean if he betrayed her, the fact she knew where he was and the fact that she had wishes with the Finns (as I have pointed out on countless occasions).