What do you think, if Faydaen had killed Marquel first, and Nicholas, the vampire, had stopped the war with the elves? It seems to me that if that had happened, the Necromancers, led by Nicholas, would have also opposed Kha-Beleh. But as it is, in my opinion, Faydaen killed a potential ally. What do you think about this?
I doubt that Nicholas would have joined the allied forces. He changed a lot after the resurrection ritual.
Each of the four who opposed Kha-Beleh had their own motives. Godric's goal was to restore the Empress to her rightful throne, Railag's goal was to save Isabel at all costs, and Faydaen and Zehir's goal was to prevent the Dark Messiah from appearing.
It is unlikely that the vampire Nicholas cared about his former fiancée or the fate of Ashan. Perhaps, after the resurrection, he even hated Isabel. After all, because of her, he was cursed, because of her, he left Erathia forever. Therefore, IMHO, the return of the queen to the throne would hardly please Nicholas.