He entered the service of the Overlord to escape his enemies. Agrail was a demon worshiper; he had no choice but to submit to Kha-Beleth. If Agrail hadn't feared the Overlord, he would have thwarted all his plans regarding Isabel long ago.

Yes, he found a wonderful way out of the situation – he saved himself, became the Overlord's right hand, and there is nothing shameful or wrong about that. However, the fact that Kha-Beleth didn't see through him reflects poorly on the Overlord.
That Agrail wasn't afraid of Kha-Beleth and even dared to defy him is clear as day, and there is much evidence of this in the Campaigns and scenarios.
And why would he want to thwart his plans? He didn't even know that Isabel was supposed to give birth to the Messiah, and his only task was surveillance. Besides, Railag/Agrail ruined many of Kha-Beleth's plans.

Still, he did submit in one instance.

Well, if so, then he didn't submit, but joined.
And, in general, the prophecy of Sar-Shazzar says that demon lords will submit to the Dark Messiah, so who submitted to whom is still a question.

To be humiliated? By whom? By his miserable subjects?

Among these "miserable subjects" there are quite a few demon lords like Agrail (i.e., ambitious ones), and couldn't they have had the idea that the Overlord was vulnerable? Wouldn't he risk the threat of a rebellion and subsequent civil war right before the birth of the Messiah?

As he said: "I will win the war."

But so far, he hasn't won anything, quite the opposite. Apart from the birth of Sareth, he has nothing to boast about.

How then do you explain the incredibly large army that Biara has in the Claw? And what about the other demon armies scattered throughout Ashan? Such armies cannot possibly recover in numbers, even by half, in just six months. Kha-Beleth has been gathering this army for over 20 years, and he couldn't have lost it in a single day.

Did I say that the entire army was defeated? And could the entire demon army be in one place? As I mentioned above, Kha-Beleth has a huge army, but it couldn't all be in Shio at the same time. The Four defeated all the troops that the Overlord had at that time, and even the Overlord of Demons cannot immediately summon the entire army.

he trusted Biara

Trusted!? Demons are not known for their trustworthiness, and the Overlord even less so, especially after Agrail's betrayal.

Arantir wouldn't have gone any further if it hadn't been for the orcs, who tied Alaric's hands.

Otherwise, would Alaric have stopped Arantir? Nonsense, he couldn't even defeat a young orc who knows nothing about magic.

Kha-Beleth was counting on Biara, in the person of Isabel, to be "serious and long-lasting."

If "serious and long-lasting" means a year at most, then his plan succeeded:rolleyes:. It is simply impossible to expect more – either Biara will be exposed, since the real Isabel appears in different corners of the Empire, or her "reforms" will simply cease to be tolerated, including by the army, and they will decide to put Frida/Duncan/some duke on the throne.

Arantir is a fanatical fool; he became a Lord not because of his superhuman strength, but because of the weakness of the other necromancers.

Well, yes, in over 100 years, no one could unite Erathia; perhaps all the necromancers were very strong, but as soon as Arantir appeared, they all suddenly became weak:rolleyes:.
Traveling through half of the lands of the Empire and the Silver Cities in battle is also not the most difficult task:rolleyes:.
And the fact that he removed the most complex spell from Isabel's spirit with a single gesture, a spell that was apparently cast by the Overlord himself, or smeared the Dark Messiah on the wall at their first meeting (as well as the not-so-weak vampire Giovanni), is this not a sign of his weakness?:rolleyes: ;)

The one who is much stronger is Markel. Markel was the most famous necromancer, the strongest of them, and, of course, the most cunning. He was the only one who could raise thousands of skeletons and dozens of dragons, and at the same time legitimize the existence of such an army, and where – in the Holy Empire! He killed the strongest mage of the Silver Cities, managed to lock Godric in a dungeon, and waged a successful war on as many as four fronts! And at the same time, he protected himself with powerful magic.

Markel is cool, no doubt about it.
But the strongest and most famous necromancer is Belket, who founded necromancy.
And in terms of cunning, Markel is number one, not only among necromancers, but among all heroes.
By the way, I felt the most sorry for him among all the deceased...:(