Do not compare Dragons to anyone. Compared to everyone else, the Overlord is all-powerful.

If he is all-powerful, then why, during as many as 6 eclipses, when he had a certain freedom of action, could he not destroy a single kingdom of Ashan? (The Empire was constantly reborn). Why couldn't he bring the Dark Messiah into the world himself, but instead waited for a millennium and the fulfillment of the prophecy of some pathetic human sorcerer (compared to the all-powerful Kha-Beleth, of course).

He is, in fact, eternal. "Although I am immortal," are his words. And the other 7 demon lords are alive, but simply deprived of power. An example of this is Azkaal. He was defeated by angels, but did not die. His soul was simply imprisoned in another soul.

His words are dictated by his high self-esteem, and of course, they cannot be taken as absolute truth. And there is nothing eternal in Ashan, except perhaps Ashi and Urgash. However, they did not exist forever either, but appeared from a cosmic egg, so nothing eternal exists.
In a sense, all races are immortal, because after death, their souls usually go to the dragon patrons, as an example, Nicholas said that he sat next to Elrath. And Sar-Elam, after dying in human form, took his place among the Dragons.

That's a bit silly.

What's silly, comparing Tiera to Yoda or Heroes 5 to Star Wars?

And the 8 demon lords (and there are indeed 8 of them) were created by Urgash, and they were apparently granted eternal life.

Where did the information about 8 lords come from? If possible, please provide a link. As far as I know, the demon lords were created in opposition to the 7 dragons of Ashi, and there were accordingly 7 of them.

The Primordial Dragons cannot be killed. But the inability to kill one's brother does not hinder omnipotence.

In my opinion, omnipotence is precisely all-mighty-ness, and if someone cannot do something, then he is not omnipotent.