This was puzzled out by others, so I take no credit.
It was definitely Graendal; and all of the clues are in the last few chapters of The Fires of Heaven.
When Moghedien is battling Nynaeve, she practically spills out the fact that Rahvin is expecting an attack, and he, along with Graendal are going to surprise him. Moghedien goes on to say something about Lanfear "plays a different game."
Fast-forward to Rand. Rahvin is not a good dreamer (this was mentioned previously), so why would he choose to fight in the World of Dreams? Because he has traps set and he is depending on Graendal’s help. Keep in mind that Rand walks by the pond pursuing Rahvin, and is attacked by fish. Later Rahvin is balefired (thus Mat and Aviendha are back), but Nynaeve still has to Heal Rand of the fish bites. Why? Because Graendal was responsible for the bites, else they would have disappeared with the balefire.
Fast-forward one hour to Asmodean. He is lounging by the "same" pond in the real world. He feels a chill (Graendal opening a gateway) and enters a nearby door which opens into a hallway. (The same door Graendal lurked in the World of Dreams to attack Rand). Graendal recognizes Asmodean and kills him on the spot.
Why didn’t Graendal just go back to Arad Doman? Well, she had to feel Nynaeve’s fire, and feared to leave right away (she may have thought it was Lanfear helping the cause). Or she may have seen Moghedien, and knowing her prowess in the World of Dreams, decided it was too risky to leave a trail for the spider to follow her to her lair. Fearing to channel, she waits about an hour, memorizes the spot, Travels to the real world. She spots Asmodean, kills him, and makes a gateway back to Arad Doman.
He is a peddler who have been twisted by the Dark One into a thing which can't rest as long as Rand is alive. He has also been infected by the Spirit Mordeth from Shaidar Logoth and now thinks about himself more or less as Mordeth. And as it looks like he does not take orders from anyone.
And how/why would he become the Dark One? It is impossible for him to become it (since the DO is not a creation from their world).
Ok. Here is perhaps an idea for discussion. What will happen to MAt in the following books? will the Daughter of the Nine Monns complete the ceremony and marry Mat? Will MAt then become the Emperor of the Seanchan once the Empress dies? And so on. Unfortunatly I can't be a part of this discussion, since my vacation starts in 2 hours. Bye.
adios even though you can't see this...well you'll see it when you get back
We know that Tuon (easier to type than Daughter of the Nine Moons) will complete the ceremony--the Finns (i forget which are which) told him already. He won't be the emperor, he'll be like Queen Elizabeth's husband.
Though I believe he will go to Seanchan to bow before the Crystal Throne; i think it was foretold somewhere (I know that Rand bowing is in the Seanchan Prophecies of the Dragon)
I think we will see a lot to Taim and the Black Tower. And probably a fight between Taim and Rand/Logain, and then Logain would be the new M'hael.
I also think that Perrin will finaly try to save Faile, up till now he has only been lurcing around searching for her. But I don't remember book ten much. Did Faile start her own escape attempt?