The strongest hero.

No one won the War of the Fifth Eclipse, although the demons were driven into a corner, but they could not be defeated.

So why didn't the all-powerful Overlord win this war, and moreover, allowed himself to be driven into a corner, since he was not fighting against the dragon gods?
Did he simply not want to?
The War of the Fourth Eclipse cannot be considered a victory, except for the capture of the capital by the demons; nothing else is known. It is quite possible that after this, they were driven back by the combined forces of the Empire's duchies, but I repeat, there is no information about the demons' further actions. And Kha-Belekh's main goal in that war was to exterminate absolutely all representatives of the Falcon dynasty, which he failed to do, and based on this alone, one can say that the demons failed in that war.

What is the benefit? To try to inflict as much damage as possible, that is the benefit. And why is the assault on Shio madness? It was stormed 3 times, and it was not stormed by fools or madmen.

The meaning of the assault on Shio can only be one: the complete destruction of the demons, which is only possible if all the countries and peoples of Ashan unite their efforts. So, the claims here are against all of Ashan, and not just the Empire, since an assault on Shio by the Empire alone would certainly be madness, which was proven by Alex Gryphon.

In fact, the Gray Alliance was defeated, the "elite of the Empire" perished, and Cyrus and Alaron fled.

No one won the War of the Fifth Eclipse, although the demons were driven into a corner, but they could not be defeated.

You contradict yourself. In one case, the demons were driven into a corner, and no one won, but in the other, it turns out that the demons, who were supposedly driven into a corner, defeated the Gray Alliance and put Alaron and Cyrus to flight.

But in relation to the rest, he is indeed all-powerful.

Again, why didn't he have enough strength and power to defeat the Gray Alliance?

But still, Kha-Belekh is eternal.

Eternal or not, but there is nothing eternal in Ashan, and even the one who created it can destroy it. So, yes, Kha-Belekh is a very strong and mighty demon, undoubtedly long-lived and not aging, but he is still not eternal in the absolute sense of the word.

No one knows how it happened that Sar-Ilam was suddenly given immense power (it was given to him, he did not create it himself)

In fact, it is known: being the first mage, he spread the doctrine that everyone can achieve the power of the Elemental Dragons, and he succeeded. He achieved the consciousness of Ashan (which, apparently, even the Elemental Dragons could not achieve) on his own, and no one helped him in this.

At that time, Kha-Belekh was an ordinary archdemon.

One of the 7 (8) original Demon Lords is an ordinary archdemon?
In terms of status, he is equal to an Elemental Dragon.
And Sar-Ilam was indeed an ordinary mortal who, as Ment noted, surpassed the Overlord in progress and power.