The 2nd Demon War

The 2nd Demon War was a world shaking event that effected all life on the material plane of existence and is currently the most influential event of mortal history. This event saw mortal life fight back against the Demonic incursions and win but at the expense of there technology being lost along with many lives.

Weakening of the Planar Barrier
At the end of the 3rd Epoch the Demons that were created were banished by The Shattering to the Abyss. The Maker knew this peace wouldn't last forever, but had withdrew at this point from intervening directly with mortal affairs ever again.

The Demons now in the Abyss only sought to escape it and bring about destruction to all life. Until the begining of the 2nd Demon War the only way to the Material Plane was through the River Styx which lead them through The Nine Hells. This is what began the Blood War.

However after fighting for hundreds of years the Demons noticed that the planar barrier that existed around the Material Plane was weakening and it would be possible to break through directly.

War Begins
The Demons strong enough the break the planar barrier now burst through into the Material Plane in droves. It was a complete surprise attack as the mortal races had no idea that this was even a possibility and thus the mortals lost almost everything very quickly. Only Major settlements survived this initial attack. This put the mortals immediately on the back foot as resources such as farms and mines were now inaccessible.

Scratching and Clawing
The Mortals now had to work with what they had left. Over the course of many years some settlements where slowly reclaimed from the Demons.

Warforged
After the deaths of so many mortal lives plenty of intelligent minded individuals were lost leaving the allied races without ways to created and maintain there current technology level, because of this the mortals had to revert to more traditional methods of fighting.

However this didn't stop those that were willing to step up to the plate to try and create a solution to one of the most detrimental problems that was causing them to lose the war.

This problem was that food and water was at and all time low and the demons could come back from the Abyss meaning they theoretically had taken no losses.

Eventually the greatest minds of this era came together and came up with a grim solution. The Warforged. The Warforged were constructs with mortal souls within them. At first peasants were taken as test subjects until they finally had a consistent design that worked. Many styles of Warforged would be created after this initial concept, but it was this moment that set the ball in motion for the mortals to make a comeback.

Devil's Deal
The Devils saw this chaos as a chance to exploit creatures for souls. Throughout the years of the Demon war the Devils would make deals that would aid the mortals, but also greatly benefit the Devils. For a high price the Devils would sell Hell Engines, Tieflings or legions of Devils into the service of mortals.

One of the greatest deals was a Pit Fiend named Koldos who bargained for a whole city to rule over, but in exchange promised to supply the forces with a high quantity of Tieflings and blacksmiths.

Reclaiming the World
Now with the Warforged on there side the mortals had soldiers that didn't sleep or need sustenance and obeyed every command. They acted as a bulwark against the tides of Demons. The Demons now began to struggle to break the enemy lines. With an unbreakable defense and a unstoppable attack the war had reached its first stalemate.

The War Comes to an End
Now with a foot hold in this war the mortals began to strategize and tactically over the course of hundreds of years kills the Demons Lords and weakened the Demons influence over the Material Planes and performed powerful to seal and restore the Planar Barrier. The final nail in the coffin was the defeat of The Watcher.

History Lost
Because of the war being a world shaking event the history was never recorded as they didn't have the resources to do so. Many of the secrets of this era died with the people who lived in it, such as; how to create Warforged and other technology such as magic items and other powerful magic.

Balor lieutenant of The Watcher
Defeated by a paladin in shinning armour.

The Watcher
Defeat by a combined effort and was locked away by a complicated ritual.

The Archon
The Archon's Demonborn were present at this time, but his plans were stopped by the mortals and even some turned to the aid of the mortals on there own free will.

Other Demon Lords
The other major Demon lords were noted to be present in many ancient texts one way or another and were defeat possibly multiple times during the war but returned once they had reformed in the Abyss

Trivia

 * In current time this event is simply referred to as "The Demon War" as mortals are unaware that there was a previous one