For a long time, the Shadow Force had been secretly constructing a superweapon known as the Binding Machine - a magical device capable of not only summoning Demons, but also bending them to their will. The machine only works by harnessing the cosmic energy that emanates from a lunar-solar alignment when the moons and suns become perfectly aligned once every thousand years. One of these alignments occurred in 400 MT allowing the invasion to begin. The Shadow Force's goal was to claim Khyorgan as a foothold for their next planned invasion on Eidyn.
The war lasted for twenty years and permanently altered the political balance of power, destroying several nations such as Gorholl and Taurya and causing the death of the Sentinel Core which had protected the plane for millennia. The war ended with the arrival of reinforcements from various Eidynese nations and culminated with the Battle of the Temple of Shadow, destroying the portal to Brunikor and causing the Binding Machine to be deactivated. The Demons, having been cut off from their mind control, rebelled against the Shadow Force and killed many of them, forcing the survivors into hiding. The Demons, however, didn't have the military strength to continue the war, and the last recorded Brunikorian army surrendered to a Polvoran army in the Battle of Tangmal City, 23 Matharian years after the start of the conflict. Most of the surviving Demons either found ways to return to Brunikor or went into hiding in Khyorgan. The Binding Machine was never recovered from the remains of the Temple of Shadow, and it's whereabouts remain unknown.
The war had a major impact on Khyorgan, wiping several nations off of the face of Erudite, and crippling others, warping once proud nations into mere shadows of their former selves. Though the war served to destroy many nations, others prospered and expanded in in it's wake; Former Tauryan holds in the Amacata Desert, such as Tangmal City, came under the control of the Kingdom of Polvora, and cooperation between various nations brought many peoples together, opening new venues of trade and cooperation.
The War of Khyorgan signified a definitive end to the Imperial Era of Khyorgan and the beginning of the Khyorganese Renaissance, a time in which trade between Khyorgan and Valordyn increased to levels far beyond anything else seen since the collapse of the interplanar empires, and the ferocity of the Shadow Force and it's otherworldy minions caused new innovation, both technologically and magically. The war and it's end created a myriad of new alliances, friendships, and partnerships, but also new contentions, rivalries, and disputes.