That the opportunity to listen to Demandred & Mesaana's plans was a reward for Graendal, because the "Nae'blis has been chosen, but there will be other positions of high glory in the Great Lord's reign. Some much higher than others. Today was a taste of the privileges you might enjoy."

Moridin, who seems actually convinced that the DO will destroy everything, if he is freed, is just giving Graendal false promises here, IMO. Like Moridin told Rand: "The only path is to follow the Great Lord and rule for a time before all things end. The others are fools. They look for grand rewards in the eternities, but there will be no eternities. Only the now, the last days."