He will win the war if Sareth sides with him. Everything has its time. And he can boast. At least that the Griffin Empire ceased to exist, the Griffin dynasty perished, and the new Unicorn Empire lies in ruins. And also, Irolan, the Silver Cities, Igg-Shail, and Grimheim were devastated. By the way, all these countries were beheaded. The results are not bad.

The Overlord only destroyed the Empire. Irolan and the Silver Cities were defeated by Markel, Grimheim was crushed by the Imperials (and not only the red ones, operating under the banner of Kha-Beleth, but also the rebels led by Frida). Igg-Shail, one could say, helped the Overlord's plans until Railag conquered it. The rulers, yes, perished, but new ones took their place. Zehir, Faydaen, Railag, Wulfsten, Frida with Duncan – in my opinion, the states did not lose anything with their arrival.

Actually, he is all-powerful. The strongest demon. He is the embodiment of Urgash.

Could you elaborate on "the embodiment of Urgash"? I don't remember that.

What if the legions are not enough to crush this quartet?
In the game itself, in mission 5 of Campaign 6, did you fight against the legions? There are very few troops there, and they don't attack all at once, but one after another. If the legions of demons, which, by the way, are always present near Ur-Hekal, participated in the battle, the outcome of the battle would be diametrically opposite.

It depends on what you consider legions. What you described are gameplay features. In fact, the legions of the alliance and the legions of demons clashed. And the demons were defeated. Perhaps this was part of the Overlord's plan, but it is unlikely that he would have deployed small forces.