I believe that Demandred is King Darlin. Demandred said in the prolouge of TGS that his rule was secured ansd he was gathering his armies. This parralells Rand appointing Darlin king and telling him to build up his army.

It works well for the Forsaken, gaining Rand's trust and eventually being appointed a position of power instead of trying to seize it for himself or sleazing his way to the throne. He can then turn his army against the Dragon as they head into the last battle.

A difficult task but surely he could do it or at least be comfident enough to try