And he is not immortal either. He is just one of the 7 demon lords, about whom nothing is heard, and who, apparently, were destroyed by Kha-Belekh himself. And if they are not immortal, then why is the Overlord immortal?
And if we are to draw parallels with Star Wars, then Tieru is more like Master Yoda, and Railag is like Luke Skywalker):D