Yes, the discussion about the Overlord is quite something. But no matter how hard some try to prove otherwise, he is not all-powerful. If even Ashu and Urgash cannot be called all-powerful in the full sense of the word, then there is no need to even talk about the Overlord.
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