In the discussion about Demandred's whereabouts I and others often suggested Murandy, especially because King Roedran is just gathering an army there. However, some people always give the spontaneous rebuttal then that Murandy is completely unimportant. I think this supposed general unimportance of Murandy isn't really backed up by the text.

Murandy is a Kingdom and much bigger than e.g. Mayene, almost as much land as Tear. There are several noble houses as well. The problem is simply that Murandy was a kind of patchwork, since the nobles didn't really care about the King.

However, if Murandy would be united, it's obviously not to neglect. For example, Gareth Bryne thinks in this context in TFoH, ch.28:
"It was his opinion that Murandy would remain a patchwork of all but independent lords and ladies until the Wheel turned and a new Age came. Murandians called themselves Lugarders or Mindeans or whatever before they named a nation. If they even bothered to name one. A lord who could unite them, and who had Siuan's leash around his throat, could bring a considerable number of men."

So, even an experienced Great Captain considers that Murandy can bring a considerable number of men, if it is united (and that would exactly be Demandred's goal, if he is there).

Besides that, there are many examples in the books, where even the not yet united Murandy is mentioned as an apparently rather important country alongside other similar important countries, e.g.:

"In Saldaea, Murandy, and Tear false Dragons ravage the land."

"Men from every land are already gathering, all eager to be part of the legend, eager to find the Horn. Murandy and Altara are on their toes, of course, thinking it's all a mask for a move against one of them."

"The War of the Hundred Years cut Artur Hawkwing's empire into many nations, large and small. The small were gobbled up by the large, or else united, like Altara and Murandy." (obviously Altara and Murandy aren't considered small nations here)

"It appears that Pedron Niall is conducting secret negotiations, trying to convince Altara and Murandy to cede land to Illian, and thus keep the Council of Nine from invading one or both."

"If the Illianer Companions had not covered the field for the rest of the army to escape Mall's trap, Altara would be a fief of the Children today, and very likely Murandy and even Illian."

"Murandy was neither far away nor unimportant." (from Elayne's PoV)

"North and east. He had to force the lands into peace, whether they wanted it or not. He had a tenuous balance in the east, with Illian, Mayene, Cairhien and Tear all under his control in one way or another. The Seanchan ruled in the south, with Altara, Amadicia and Tarabon under their control. Murandy might soon be theirs, if they were pressing in that direction. That left Andor and Elayne." (from Rand's PoV)

All this indicates that Murandy certainly isn't one of the most powerful nations (e.g. Illian and Andor seem generally more powerful), but it's certainly comparable to Altara, Amadicia, Tarabon or Ghealdan, and thus not unimportant at all.

If one adds then the idea that Demandred was previously additionally behind most of the chaotic Dragonsworn in the countries of the south-west of the Westlands (namely in Arad Doman, Tarabon, Ghealdan, Amadicia, Altara and Murandy), Demandred managed quite a lot, if he also has Murandy as a nation and its King under his thumb.

It's IMO at least not a worse accomplishment than only having Taim & a hundred DF Asha'man at the BT under his control, as other people suggested. This is especially true, if one of the Shadow's main goals is to prevent that Rand unites all of the Westlands against the Shadow and to create chaos.

Finally, I suggest that Demandred might be a noble or general at King Roedran's court (this would explain Demandred's clothes). I think he might pose as a member from House Chiendelna. We know that Luc posed as a Lord of the minor but old Murandian House Chiendelna. Maybe this is a noble house, which was "overtaken" by the Shadow (similar to the Andoran House Avarhin, whose members were killed by Mili Skane, before she took the identity Lady Shiaine Avarhin). Demandred created then a new ater-ego as a member of House Chiendelna as well to infiltrate King Roedran's court. Otherwise Demandred might be a powerful Dragonsworn leader, who was eventually raised by the King to become a general.