"The best army in the world would have Damryan footsoldiers, Lancesian cavalry, Ibarrish archers, Anderian siege engineers, Calavrian officers, and Norlish scouts.
"Unfortunately, they wouldn't have anyone to fight but each other."
The Realm is the union of the nations of humanity. It was founded by Damryn upon the conquest of Lancereaux and Andermark, and over the centuries would expand to include Calavria, Ibarran, and eventually Norlundar. Formally the latter five are all client states of Damryn, even though Andermark at least continued to keep a king in its own right.
The High King of Damryn is officially the leader of the Realm, and all rulers and subjects of other nations formally owe him their loyalty. In practice, the unity of the Realm applied more to matters military and economic rather than social or political. Damryan Law did not affect the Realm as a whole unless the High King issued a specific edict to say it did. Beyond the boundaries of these universal laws, the subordinate nations were free to govern themselves as they so chose. Realm laws extended to such matters as important as the banning of human slaves, and mundane (though integral) as a universal calendar.
Economically it was still preferable to conduct trade within the Realm than without, as discovered by Calavria, which joined the Realm specifically to avoid economic sanctions placed upon it by High King Donan. Taxes for trade inside the Realm were considerably less than they were for trade with the Iron Empire or Sahradia, and the Realm so benefited from the free-flowing of necessary goods to lands which could not necessarily produce them for themselves that eventually the economic advantages even included government subsidies for industries perceived as necessary, such as Andermark's lumber trade. The easy transport and trade of goods made for a flourishing economic situation across the Realm, though through taxation it was Damryn who benefited the most from the economic boons.
The greatest advantages of the Realm ostensibly came from the military alliance. While if Damryn raised its banners and demanded the aid of the Realm, the other nations were obligated to lend aid, this did go two ways. Although the War of the Narrow Sea was started by Damryn and became a centuries-long conflict with the Iron Empire, its roots lay in tensions between the Empire and Calavria - tensions and threats Calavria could not contend with alone. Additionally the Crusades against the Sahradians were matters which occupied the whole of the Realm.
These Crusades also won the High Kings, and the entity of the Realm, the support of the United Church. Consequentially the clergy worked hard to ensure the people of the Realm supported and endorsed the alliance, even if it was not always advantageous for an individual nation. This would go a long way to ensuring the people of the Realm accepted the rule of Damryn over and above their own kings or liege lords.
In practice, few of the client kingdoms had issue with the Realm. Andermark had not lost a great deal, and ultimately flourished from the freedom of movement both economically and religiously. Ibarran joined the Realm after Damryn led an army to expel Sahradian occupation and enjoyed a subsequent stability it had never before achieved. Calavria, although economically strong-armed, only became wealthier and more influential from the Realm's existence, and Norlundar joined specifically to bring about the end of tensions and raids on the border with Damryn which stretched back to time immemorial.
The only nation which arguably did not flourish under the Realm was Lancereaux, though the kingdom did benefit economically enormously due to its low overheads in its agriculture and a bigger market to sell to. But as the first nation to fall to Damryn they lost their king and were forever seen as more subservient than other nations. Calavria, Ibarran and Norlunder never contended with Damryn; they had their own focuses, be they economic, religious, or simple survival. Andermark, on the other hand, had been allowed to keep its king and the difficulty of travel across the nation meant it saw less everyday Damryan interference. Lancereaux placed a lot of stock in its pride and its status, and Damryn seemed to take a perverse delight in undermining both of these at any given opportunity.
But the Realm survived until the Fall, though the survivors of the nations still formally consider themselves citizens of the Realm. The question will doubtless arise as to whether the Realm itself will be restored when borders are expanded beyond Andermark and, if so, what form will it take? Will there be a High King? Will that High King be Damryan? These answers are not yet needed - but they are not forthcoming, either, and the day will soon come.