Well.. honestly I think it's pretty clear Demandred is under Moridin the mastermind of the War. His role will be very prominent (he'll be the main foe of Mat, notably), and I believe Brandon thinks the reveal of Demandred's plans will be huge because most people still don't adhere to theories that Demandred isn't infiltrated anywhere and still insist on looking for him among the whole list of secondary characters.

The big mystery left is what sort of armies he will bring in. I expect it will be impressive, mostly because his armies will be run on far more modern principles than anything else in the series. They may not be that huge, but they'll wipe the floor in terms of tactics, logistics and global organization. Demandred knows what modern armies need, he'll have devised all sort of tricks to get around the absence of ter'angreal (eg: his officers will be trained in weaves to communicate from afar and stuff like that), they'll have well stocked hidden food reserves reachable by Gateways, healers and so on. No doubt Demandred will also have protocols for every situations in place... his armies won't be chaotic.

There isn't a whole lot of new stuff about Demandred in the prologue, except Moridin specifically mentions for the first time his role in the war, by telling Moghedien Demandred has plans for the war and needs her for an army that now needs monitoring (thus my speculation that Moghedien is needed by Demandred to take over from Mesaana.. though it's also possible Moridin is imposing Moghedien on Demandred).

We learn Moridin is summoning Demandred every other day, and Demandred isn't pleased to be taken away from his preparations.

We learn from Moridin (and safely out of Demandred's earshot!) and in the context of reminding Moghedien of her failures and that she's still not completely forgiven and only in probation (she's got her mindtrap back) that the Dark One is pleased, very pleased with Demandred's doings. In Demandred's presence Moridin is not exactly flattering... calling his armies his "playthings", barking at him that unless Demandred has been named Nae'blis in his back he must shut up and obey, the usual show of authority.

Demandred is dead silent when Moridin introduces Taim as a New Chosen. He has no reaction at all, but shortly after when Moridin boasts of Taim's achievements and reminds the others they don't have much to match this, Demandred explodes and yells at Moridin he will see the results of his plans by bushels and droves but he's better remember what's his price for that, which is that he will be the one to face and kill Rand. He then gives a challenging look to each Chosen including Taim. It really sounds as if Moridin has bought Demandred's loyalty seen in the last books by the formal promise he'd be the one killing Rand.

I very much doubt Demandred still plans to go after Moridin or replace him as Nae'blis (he probably postponed that to after the War, Demandred is I think perhaps the most naive of the lot when it comes to Shai'tan's true intentions). He's got what he wants. Brandon pointed out earlier Demandred isn't nearly as interested in Moridin's place as he is in Rand. At this point being Nae'blis would almost be detrimental to Demandred, it's fairly clear what he's completely eager to do is to lead the War (which, by the way, Moghedien has coined as "(a new) War of Shadow". He couldn't do that and take over from Moridin who seems to have tons and tons of initiatives of his own. It's too much for one man, and it's too late to reorganize everything - there's just not enough Chosen to rule over either. I'm sure Demandred's intelligent enough to have figured out that on his own, without even his two allies to delegate tasks if he were Nae'blis, he would be shooting himself in the foot if he took out Moridin.

Moghedien wonders what has changed lately in Demandred for him to want to kill Rand himself, apparently Demandred had never cared about that before, only that LTT/Rand died. I think it's pretty obvious what has angered Demandred is the loss right on the eve of the War of Semirhage at Rand's hands, and Mesaana at the hands of Rand's friends, which must have suddenly made this very personal for Demandred. Mesaana is probably the worst loss of the two, as Demandred was probably counting on her in his plans to defeat Tar Valon down the line, and plans probably had been updated already following Semirhage's demise; His plans probably took into account the presence of Mesaana and an army of several hundred BA inside ready to betray the Tower. I suspect the loss of the Seanchan isn't nearly as bad, as they clearly still intend to fight the Westlands whether Semirhage is there or not to push and pull Tuon. That will change if and when an alliance between the two sides is made...

When you think about it, Demandred really had it served on a silver platter: the BT was taken care of, the WT was split and Mesaana had several hundreds BA still inside, the Seanchan were to keep the Westlands busy from behind... At the beginning of TGS, this looked like it would be a war against a bunch of mundane if strong armies, with the BT nearby the biggest, the armies of the Borderlands stuck at Far Madding, Rand's armies scattered all over the place... so he basically had his inner circle with Logain's Asha'man and Cadsuane's troops, the Wise Ones and the Aiel, and probably the Windfinders.

Now it's more of a challenge... Mat has gone to Tuon, Perrin's group's not nearly as weak as before and he's ta'veren, Rand is super Rand, and the odds are great Rand will have a few thousand female channelers behind him... WO, Windfinders, Aes Sedai, their novices and accepted etc. And for the moment, all of these will be gathered at Merrilor for a time.

Rather than a contest between Moridin and Demandred (though it's clear Demandred isn't trusting Moridin won't betray him) what Moghedien foreshadowed is that Demandred has better watch his back with Taim. She's probably right about that. The bigger threat to Moridin and his plans (and to Demandred's hopes to face Rand, for that matter), IMHO, is Cyndane. That one's ruled by her passions and grudges and she has little of Demandred's pragmatism.

IMO, Cyndane is now trying to kill Rand in order to kill Moridin at the same time.

One last note on Demandred: I don't expect him to be "infiltrated" because it would suppose he will bring armies to the war on a false pretext. Sure, he apparently loves doing that apparently and it's quite possible he's got leverage to start a few proxy wars as distractions (eg: bringing a Sharan army to attack the Westlands etc.) but if that were his forces that's way too risky: these armies wouldn't serve the Shadow, and they can switch side. I'm sure he's got armies of dedicated Shadow followers, and that he will have control of most of the Shadowspawn armies available (and his people will be trained to fight with those, and lead them).

I'm pretty sure we've already seen some of Demandred's people at work in TOM (they were the ones operating the Portal Stones, IMO).