Prediction, Akka and Mimara find Seswatha himself in the bowels of Ishual. Through sorcerous means, he has lived for 2000 years without his heart in the part of Ishual where no light is allowed. There he directs the Dunyain breeding program without the Dunyain's knowledge. For example, every Spring, the Dunyain dump the new crop of candidates in the maze, one month later a few survivors straggle out.
OR! Seswatha is in suspended animation. Through a series of hijinks, Akka gets the heart and crams it in the old sorcerer's chest. Seswatha reawakens just as Mog is reborn!!!
EDIT: Maybe we'll see Seswatha return like the shade of Gin'yursis??? Possess Akka!