War of Souls
This is the name given to the civil war that destroyed the western Noefran empire, Tibor Imperia.
Prelude
After the Aurum family took the imperial throne in 997 BC through assassination, the decay of the moral fabric of the empire accelerated. The weak Aurum rulers, more concerned with parties and flashy displays than actually ruling, were unable to prevent the drawing-apart of the provinces. In addition, the demonic-prompted research into blood magic, into using souls as fuel and grafting souls onto others to create artificial life was allowed to run without check. Slavery, once banned in the Empire, was re-established. First as a punishment for debt, then wholesale. Any form of justice for noble crimes diminished outside of a few remote provinces.
The Last Straw
When the dragonborn were unveiled as the core of the new Imperial Army in 800 BC, the revulsion to the method of their creation was widespread. On the occasion of the marriage of Emperor Aurum XVI to his 12th wife (a girl of 10 years), three members of the emperor's Oathguard and 24 others sacrificed themselves to detonate the largest concept bomb ever. It removed a 14-mile-radius sphere encompassing the entire Imperial capital from existence, along with the entire Imperial Senate (except 5 members) and all but two of the imperial family.
War
The resulting chaos spread quickly. Province against province, city against city. Histories of exactly who the factions were or about the battles have mostly been lost--the use of concept bombs was commonplace and much of the landscape has changed. Sporadic bouts of war lasted nearly 200 years. Areas would be at peace for generations, then suffer a spasm of war. Overall, tens of millions of lives were lost and the demons feasted.
Aftermath
In the wake of the War of Souls, the central region of the empire was almost entirely depopulated. Civilization only survived around the edges. It took 600 years to rebuild, and technology never reached the same heights.