Shirone Kingdom is a small country, but it's an old country that has 200 years of history.
400 years ago, all human race countries except Asura Kingdom and the Holy Milis Kingdom were wiped out in the war.
Until 300 years ago when Dragon King Kingdom took control of the southern-most tip, it was a region of intense strife.
Even now, if you go north, the strife zone spreads out.
The Shirone Kingdom is a country that is relatively close to that strife zone.
Shirone Kingdom manage to last as a country for 200 years because they formed an alliance with the Dragon King Kingdom early on in their history. Even if you were to call it an alliance, the difference in power is clear.
If you were to stop by the Shirone Kingdom on the way to the Dragon King Kingdom it's like the other two kingdoms, a vassal state.
It revealed that the king of Shirone Kingdom refuse to give another hostage to Kingdragon instead Pax, which will marry to Benedict, 18th princess of Kingdom of Dragon King. For the King of KDK, Shirone is just a vassal states.
To show his wrath, the king sent his most able knights, Death God Randolph and the Order of Black Knights, with Pax to initiate a coup.
They murdered the rest of the royal family, and installed Pax on that still bloody throne.