First off, she was more than likely still in Finnland, or busy having her soul brought back by the DO and put into Cyndane's body.

Second, if she was out, then she she looked like Cyndane, and Asmodean wouldn't have recognized her, so the whole "You?" comment makes no sense.

As far as I'm concerned, it looks like Graendal with Slayer 2nd. However, Slayer is the Shadow's assassin (Joiya/Amico, tried to get Fain, tried to get Rand/Min), and we know from the interviews that the DO did NOT order his death. Coupled with the fact there was no body discovered and the fact that the DO's talk with Demandred implies a balefire use (he who betrays me dies the final death), though this isn't certain.

Of the Forsaken:
1. Granedal. Probably. Knew Rahvin was in Caemlyn and had been plotting with him. Knew about Asmo going over from Lanfear in the Lanfear/Rahvin/Sammael group. Motive a bit weak, but had opportunity.

2. Sammael. Doublful. Again, likely knew Rahvin's location and Asmo's defection. Motive suspect and not his style, but maybe he thought he couldn't afford to have Asmo teaching him if he did confront Rand. However, seems not to be his style (he liked to defend) and he seemed to not know Asmodaen's fate when talking to Graendal, but could have lied. However, with him dead now (per RJ), how do we get a reveal with him dead? Did he tell someone?

3. Mesaana. Possibly. Doesn't seem like her style. We know Moggy knew Rand was going after Rahvin that day (and his location), how she knew I have no idea, but did Mesaana? We know Moggy was talking with Graendal, and that Moggy was watching the meetings in T'A'R, and was watching Elayne/Nynaeve, so maybe she had her eye on Caemlyn or Cairhien? But how would Mesaana know? Not a T'A'R master like Moggy/Lanfear/Ishy.

4. Semirhage. Possibly. Same as Mesaana.

5. Osan'gar/Aginor. Almost certainly not. Again, we have no sign that anyone knew about him coming back (Lanfear's comment about nearly half the remaining Chosen in one place when she/Graenal/Sammy/Rahvin met. And if he wouldn't be recognized. And we don't know if he was back yet. And like Sammeal, he appears to be dead now, so how'd we get reveal? Though he could be brought back again.

6. Aran'gar/Balthamel. Almost certainly not. See above. Plus there is a reveal problem here as well.

7. Moridin/Ishy. Almost certainly not. See above.

8. Lanfear. No. Knew the location and had motive, but no opportunity (dead/in Finnland) and Asmo wouldn't have recognized Cyndane.

9. Moghedien. No. We know from Demandred's PoV starting LoC that she was meeting with Graendal, so could have known Rahvin's location (and thus that Asmo was with Rand if he went after Rahvin). But no opportunity. Nynaeve drugged her at the end of tFoH and she was on an adam leash after that.

10. Demandred. No. We know from his PoV in LoC he didn't know Asmodean's fate.

11. Rahvin. Impossible. Balefired.

12. Be'lal. Impossible. Balefired.

13. Slayer. Possible. RJ said all the Chosen knew Slayer, and vice versa. He has unique T'A'R abilities that could have gotten him there, and figured out info about the Lanfear/Rahvin plan. Could he have pulled the body into T'A'R with him and exited at another spot? Seems like it. His Luc persona would certainly know his way around Caemlyn. Still, he's the Shadow's assassin, and we know the DO didn't call for a hit.

14. Fain. Doubtful Asmo would know him, or vice versa.
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A channeler would need to know traveling and balefire, and that rules just about everyone else out at that point.