Vernium itself is large, and if he compared it to the Human World kingdom, it would be even larger than Vern but fortunately, the governance system is not that organized yet.
And their population is scarce.
Tribal lords and Warchief does not always follow the word of the King here.
If this happens in his Empire, with a rebellion, some people would still hold the banner of his family, those who swore fealty to him with uphold that honor
However, the tribes and the people of the Shadowlands, never regarded the King to be the true king. To many of the people of the people of tribes, their true King has always been their prophesized Asrana.
So, if Arial could convince the Shadowlands that he is Asrana, the resistance would fall almost immediately. It would not be as harsh or as difficult as when Arial began the Conquest War.
Of course, convincing people that he is their savior is not something that easy.