Three Kingdoms IV

In conjunction with strictly imposed discipline and a new code of laws, regulating all aspect of army life, Arial transformed these primitive tribesmen into his sharp terrifying blade.

The reason why Arial maintain such high discipline in his army is because the very nature of the tribesmen who respected the strong. 

They are obedient in regards to their leaders, even more so than to their own family. 

They hold their leaders in the greatest reverence and never tell them a lie. As long Arial won their admiration, then they would even sacrifice their life for him.

It reminds Arial many of the values of Orcs.

The second reason why Arial remains undefeated in his campaign to reach the Shadowlands is because the leaders on each level had significant license to execute their orders in the way they considered best.