Moghedien knows some very sensitive information, knowing how she got it may have significance to events that occur in Caemlyn.

“Al'Thor means to go after Rahvin. Today. This morning. Because he thinks Rahvin killed Morgase. I don't know whether he did or not, but al'Thor believes it. But Rahvin never trusted Lanfear. He never trusted any of them. Why should he? He thought it all might be some trap set for him, so he has laid a trap of his own. He has set Wards through Caemlyn so if a man channels a spark he will know. Al'Thor will walk right into it. He almost certainly already has. I think he meant to leave Cairhien right after sunrise. I had no part of it.”

TFoH. Ch54. ‘To Caemlyn'

This passage shows that Moghedien has had some very recent and accurate information on both the actions of a fellow forsaken and on the plans of Rand.

So the questions are.

1- How does Moghedien know Rand means to attack Rahvin? (Cairhien)

2- How does she know of Rahvin's preparations? (Caemlyn)

Firstly from the Moghedien side of it. How has Moghedien been gathering information? There's no real possibility that she has been to either Cairhien or Caemlyn personally as she is caught up in her personal vendetta with Nynaeve and Birgitte and is posing as Maigan. It would be foolhardy and very unlike the ‘spider' to go Gateing around especially from a Salidar full of Aes Sedai to a Caihien full of the Dragon Reborn.

The second possibility is that Moghedien has spies in either or both locations. This again would seem unlikely; she had her center of operations in Tanchico, a long way distant from both Caemlyn and Cairhien.

The main route of information for Moghedien has been spying in Ter'aran'rhiod, Nynaeve and Birgitte have seen her watching Lanfear, Rahvin, Sammael and Graendal.

‘Below them, Moghedien also stood on darkness, garbed nearly as black as what surrounded her, bent and listening intently. And as far below her, four huge, high backed chairs, each different, sat on an expanse of glistening white-tiled floor floating in the blackness. Strangely, Nynaeve could hear what those in the chairs said as well as if she had been among them'

TFoH. Ch 34. A Silver Arrow

So it is possible that Moghedien has seen people discussing the information. However it is hard to see what group of people she could have overheard. The only grouping that may work I'll consider right at the end.

The most likely source of the information for Moghedien is that someone told her directly. Let's consider the timing.

The news of Morgase's ‘death' and the decision to attack Rahvin the next morning is made sometime in the afternoon / evening and at Moiraine's urging is set for the following morning. Next morning Lanfear and Moiraine take their tumble through the ter'angreal on the docks. Then Rand leaves with the chosen Aiel, Aviendha, Mat and Asmodean for Caemlyn. At roughly the same time Nynaeve captures Moghedien. So there was only one night for that information to spread.

We can eliminate Moghedien overseeing any meeting between the good guys in Ter'aran'rhiod because no such meeting took place. We can almost certainly discount Asmodean as it would not be in his best interests. So we can probably assume that there is either another spy or that there is a Forsaken in the nest. Whatever the primary source of the information for Rand's intention it was almost certainly passed on by another Forsaken to Moghedien.

Further I think we can narrow it down to four, those directly involved in the Illian plot as they were the ones most involved with the events. I'll cover each of these in turn.....

Lanfear. She was in Cairhien witness the confrontation at the docks and she was the instigator of the Illian plot (TFoH Prologue) so she was in a position to find out about Rand's plan. But why would she pass any information about Rand to Moghedien. Also why would she be discussing any such received information with anyone else and not with Rahvin and the others involved in the plot? It also makes no sense that she would know such potentially explosive information and then wander down to the Cairhien docks to check on Kadere. Specifically a quote:

‘By showing your obedience to my commands. I have been too busy to watch Rand al'Thor myself. Tell me what he has done, aside from conquering Cairhien, what he plans to do'

TFoH. Ch52. ‘Choices'

Sammael. He does have a spy in Cairhien (if we assume Melindhra was sent by him) however she dies before she can report anything. Sammael is actually shown depending on others to do his spying:

‘“Have you learned where al'Thor is hiding Asmodean? Or anything of Lanfear's whereabouts? Or Moghedien's?.....”'

‘“You know as much as I do” Graendal said blithely.'

LoC. Ch6. ‘Threads Woven of Shadow'

On the subject of Rahvin's preparations. Sammael would not have been able to visit there to discover them as they would have been triggered by his channeling. There would also be no real reason for Rahvin to tell him. Sammael was concentrating on preparing the trap in Illian in expectation of Rand's attack. Why would he have told Moghedien?

Rahvin. Well he definitely knew about his own preparations and was suspicious of his allies fearing an attack. However if he found out about Rand's intentions would he have been alone in Caemlyn? Was he alone in Caemlyn? As with Sammael why would he have told Moghedien?

Graendal. Now there is no direct information that shows Graendal was the source of Moghedien's information, but she has one advantage over all the other candidates. She did meet Moghedien.

‘“Rahvin is dead, Great Lord. Yesterday.” There was pain. Euphoria too strong became pain quickly. His arms and legs twitched. He was sweating, now. “Lanfear has vanished without a trace, just as Asmodean did. And Graendal says Moghedien failed to meet her as they had agreed. Also yesterday, Great Lord. I do not believe in coincidence”' LoC Prologue.

Two points from this.

1- Moghedien and Graendal had met and agreed to meet again

2- Graendal met with Demandred

In addition, of all the Forsaken, Graendal is the one that speaks to the most of the others. It would not be surprising if she had had one on ones with Rahvin and thus learned about his preparations. From Sammael's demand for information from her (see above) it seems as she has good sources (probably from those under Compulsion) and could have agents in Cairhien, perhaps even among the Aiel. A problem as with Sammael and Rahvin is why she would of told Moghedien? But she's always liked having her fingers in many pies.

Rahvin and Graendal Moghedien could have overheard a meeting between these two, but as she was meeting Graendal anyway it seems a little less likely then Graendal just telling her.

So this is my theory

1- Rahvin prepares his defenses.

2- He tells Graendal of his concerns. Along the lines of ‘I'll be damned if I get caught out'

3- Rand hears of Morgase's ‘death' and decides to attack Rahvin.

4- The information passed to the Aiel societies and through some route to Graendal's ears sometime over the evening / morning.

5- Graendal is meeting with Moghedien in Ter'aran'rhiod and passes over the information

6- (Lanfear / Moiraine at the docks)

7- Moghedien is captured and misses next meeting

8- (Rahvin balefired)

9- Graendal meets Demandred.

Now what was the point of all this. Well if Graendal has the sources to know what the Dragon Reborn plans to do within 12 hours of him deciding, what else does she know. Did she know that the Dragon Reborn's faithfully gleeman was accompanying him? Does she know who Natael is? Was it Graendal who not only passed on the information of Moghedien's non appearance to Demandred but also the news of Rahvin's death? Did she in fact also report that to the DO at her own meeting?

‘Only she herself knew that she had made her own journey to Shayol Ghul and down to the lake of fire. Only she knew that the Great Lord had all but promised to name her Nae'blis,'

LoC. Ch 6.Threads Woven in Shadow.

Was Graendal in Caemlyn? Did she meet an old friend, briefly.