to accomplish what you are talking about. The training alone would be almost impossible to achieve. even the BT only has a few dozen full Asha'man. Channeling is a skill that takes decades for most to master, creating an army of warriors from scratch would take a lot more time than he has had.

And to manage something like that in total secrecy seems nearly impossible. Eventually people will start to notice recruitment or strange disappearances. With the WT, BT and Seanchan already out recruiting it would be that much more difficult to build in mass numbers. Taim's men alone seem to have hit just about every village and town on the continent and stripped regions bare of men. Egwene and crew aren't that far behind now that rules are changing. And Demandred would be at a deeper disadvantage in that he would need to screen for people with a predilection toward the Shadow, that or force convert them, which takes resources and time and draws the attention of the other Chosen. Perhaps he has been on the Isle of Madmen and/or Shara/Seanchan recruiting ... Would at least explain his being so elusive to the other Forsaken.

Don't get me wrong, I like the concept to a point, but I don't see how he could have kept it hidden from the other Chosen as well as the Lightsiders for so long. And we know Moghedien and Graendal were both actively trying to discover what he was doing. It doesn't even sound like Moridin fully knows what Demandred is up to ...

“You will know the fruits I have harvested, Moridin,” Demandred said, voice low. “You will know them in bushels and droves. Just remember my requirement: I face al’Thor on the field of battle. His blood is mine, and no one else’s.”

Note he says you WILL know, not you DO know. I think Demandred is still relatively free to do as he wishes since he hasn't screwed anything up yet. My guess is that Moridin has far less control over him than he would like or they wouldn't be bargaining.