The Coup of the Heretical Kings
Prior to this Coup the Alliance of the Three Kingdoms were all stable and prosperous kingdoms. They shared good diplomatic and trade relations with one another because they shared a common religion. In the months before the coup one of the kings had been trying to convince the other two rulers to join him in leading a crusade across the land, the other two kings declined.
The King invited the other two rulers to his kingdom to attend a festival celebrating their shared god. While this was taking place the King mobilized his army, dividing them into three groups: the section in the home kingdom, and the two section sent to the other two kingdoms respectively. Up until the point the three kingdoms had no hostility toward one another so the other two kings did not mobilize any force other than their personal guards for when they were traveling.
The two Kings were immediately apprehended, they were dressed and devils and paraded around the kingdom as heretics. After a day of this the two kings were showcased in the stronghold of the kingdom while citizens pelted them different items. Finally they were publicly executed and their bodies hung on the outer wall. At the same time this was happening the other two forces raided the respective kingdoms, the guard were unprepared and slaughtered as a result. As the armies' invaded all those who put up resistance were detained or executed. Few members of the royal families managed to get away and those who didn't were subjected to similar treatment as their kings. Any civilian who didn't take part in the public shaming were executed along with the remaining royal family.
The consolidation of these three kingdoms into one drastically increased the power of the individual kingdom. This power would eventually lead for expansion of territory leading to the formation of the Holy Empire.