The Siege of the Northern Holy Kingdom was an extensive conflict instigated by Jaldabaoth invading the Roble Holy Kingdom using an army of demi-humans from the Abelion Hills and laying siege to its northern regions. Following the initial fall of the Holy Kingdom, the conflict became a large-scale rebellion against the demi-human occupiers.
Not long after Nazarick had been teleported to the new world, Demiurge thinking that Ainz's mention of conquering the world as something absolute, he started preparations in order to achieve his master's goal and had set up a base of operations in the Abelion Hills in order to first harvest materials and to conduct his experiments for the benefit of nazarick, it was at this time his forces have been subjugating the tribal demi-human races that lived in this region as well as kidnapping several citizens within the Holy Kingdom to be used in the production of items such as magic scrolls or used in breeding experiments, some of the demi-human races joined willingly others under the threat of force and began plans to commence the siege of the Holy Kingdom. He assigned all the strongest demi-humans of each race as generals under his command and appointed three of Demons to be his commanders, with Rokesh the Naga Raja being the one directly to speak between him and the demi-human leaders. Once everything had been prepared the invasion of the Holy Kingdom had begun under the disguise of using his Jaldabaoth persona to begin the full invasion.
In the middle of the night, the Demi-human alliance led by Jaldabaoth greeted the soldiers at the Great Wall in to order to begin the siege of the Holy Kingdom, While everyone had shot arrows and magical enchanted spells towards the demon emperor, it proved to be ineffective and with one spell, the Wall was breached in a fiery explosion and almost all within its AOE were killed of instantly and thus the demi-humans had poured inside and the invasion had officially begun.
News of the wall being breached spread quickly and a mobilization order was given to entirety of the Holy Kingdom. With the demi-human army heading towards the city of Kalinsha in the Northern Holy Kingdom, the Holy Kingdom's stratagy was for the Northern Holy Kingdom Army to hold out until the Southern Holy Kingdom Army could arrive and incircle the invaders.
However, Jaldabaoth proved to be a step ahead, as he leaked false information as to his army's pace. As a result, the demi-humans arrived at Kalinsha much sooner than expected, with Jaldabaoth attacking the city by himself. The Holy Queen was inside the city as this was happening and, not left with many other options, decided to rally her personal forces to face the demon. Unfortunately, they were ultimately no match for him and the Demi-Human army successfully conquered Kalinsha, with the Holy Queen being taken as a prisoner and later killed.
Afterward, the demi-human army moved on to conquer most of the North, with citizens being taken to prison camps for experimentation. However, when they attempted to invade the Southern Holy Kingdom as well, their forces were finally halted by the Southern Army at the border.