Brass Concordat

When the Abyssal Gate was opened in the Imperial City in A.I. 400 it unleashed armies of demons on the continent. On the first day the demons consumed the capital of the Imperium along with the God-Emperor and the empire's entire leadership. This prompted the various provinces to revolt and declare their independence, starting the destructive Succession Wars as the newly formed realms fought over territory. In the meantime the remnants of the Imperial Legion tried and failed to hold the innumerable demons at bay. The new kingdoms soon realized that winning their own little wars would mean nothing if the demon horde is not driven back.

The young realms put aside their differences and joined together to push the demons back but even their combined force was not enough to stop the demonic onslaught. In this darkest hour help arrived from an unexpected source. The Lords of the Nine Hells reached out to the kings: they offered to help push the demons back and keep them at bay (In D&D demons are chaotic evil beings living in the Abyss who are constantly at war with devils the lawful evil beings who live in the Nine Hells). In exchange they asked for access to the mortal world, that they be allowed to establish embassies where any mortal could commune and make deals with them.

With no other choice left all realms agreed to the conditions known as the Brass Concordat (named after the Brass Citadel in infernal plane of Avernus where the treaty was signed). Fiends and fallen angels joined the armies of mortals and together they drove the demons back to the Imperial City. But the Abyssal Gate could not be closed, so through magical means a system of forts and a large wall was built around the area. This Demonfence is what stands between the people of the Aegeron and the ravenous hordes roaming the ruins of the imperial capital. The Demonfence is manned by the remnants of the Imperial Legion, now only a pale shell of its old self. The legionaries of old are gone (few elf and dwarf officers still live), the legionaries of today are foolish volunteers and criminals the Young Realms banish to the Demonfence (think of it like the Night's Watch from Game of Thrones).