Moridin and Demandred have always been quite close as the highest members of the Shadow, and while Moridin has always been present and incomprehensible, Demandred appears to be absent while working his plans behind the scenes.

Ishamael has been visible from the beginning of the story. In the War of Power he was the leader of the Shadow and was defeated by LTT. In the 3rd Age he tried to convert Rand and ended up seriously wounded twice, and finally killed. The other Forsaken who died who failed were reincarnated with strict penalties usually in the form of servitude, though Moridin seemed to evade this despite his incompetence being no less than his peers'.

Is it possible that he's been duped by the DO? The point of making a false Nae'blis would be to bring the other Forsaken into line for the Last Battle, which Moridin apparently has done. He's undoubtedly insane, and his orders sometimes countermand themselves, so perhaps that's all he's good for. As an individual Forsaken, he's got almost nothing to show for it.

Demandred, on the other hand, could be the true Nae'blis. He was promised the title in LoC, but then Graendal insisted that she was too. However, hints about his prowess and the fact that he would be Dragon if not for LTT are strong hints that his role in the Final Battle could be monumental. It is also rather telling that the DO used Demandred to spread the LoC message to the other Forsaken, even though the concept is old to the Shadow.

As an alternative hypothesis, I would propose that Moridin is a pseudo Nae'blis whose only function has been to organize the highest levels of the Shadow. When the Last Battle is initiated the DO may kill Moridin and his conveniently linked uber-useless slaves in the form of Moghedien and Cyndane, and hand over the Shadow and the True Power to Demandred. Alternatively he may place Moridin under Demandred, who will attack the Light with the organized remaining Forsaken, as well as Demandred's organized armies that he's been rounding up for a few years.

Thoughts?