The Smiting

The single most significant event in Ayndrinor's history since creation, The Smiting essentially reset development and decimated the humanoid population of the world right at the turn of the fourth millennia in the First Age. The First Age was characterized by widespread warfare and violence between different city-states and small regional kingdoms. Very few alliances were formed - except those small regional kingdoms - and suspicion and xenophobia were rampant.
As time progressed, these constant wars led to the development of several massive supercities and their creation of devastating weapons, both arcane and mechanical, that could be used to wipe entire cities off the map. A few were used in warfare, though most were held as deterrents. The Gods of the Light Pantheon saw this happening and realized the world and the sentient civilizations they had created were in grave danger of being destroyed. So, they banded together and did something that most would have considered unthinkable to that point: they took the world back to the early days in the hopes of humanity going a different way.
The first thing to go were the massive supercities. As the most egregious violators of the gods' trust, these cities were completely erased from existence, along with their entire populations and any external people or documents that mentioned them. This left entire geographical regions with few or no settlements - most notably the eastern half of Ayndrinor that has since become the Everyn Federation. All evidence of advanced arcane research was likewise erased from existence, and future generations would have to rediscover many lost secrets.
The next thing the gods did was to decimate the remaining populations of the cities and towns, leaving only ten percent of each settlement alive - and an additional 20,000 people they randomly resettled. They made sure this step did not lead to the extinction of any races, to preserve every group that existed at that time. The remaining populations had their memories wiped and could not remember anything about the world before. They "awoke" in massive cities with very few inhabitants, knowing something had occurred but not sure exactly what. The gods allowed histories and archives to remain untouched - except where they mentioned the presence of the supercities or what had existed before. In this way, the remaining people were able to preserve their histories and have the knowledge of when things were built and by whom.
The gods decided to help expedite the resettling of the supercities' lands by taking the 20,000 people and randomly dispersing them across the decimated areas. These people "awoke" in a strange land with only a few hundred other people around and no villages or towns around, with no memory of past lives or the rest of the world. Likewise, these people were stricken completely from the memories of the rest of the world's population, leaving no connection or coincidental meeting possible. These relocated people began to spread out from their various "spawning" areas and look for places to settle and found new cities. Many great cities of the world were created during this time, with little influence from the remaining cities, who were too busy rebuilding themselves and creating new cultures and technologies to venture across the world to distant lands.
The unintended consequence of The Smiting was the weakening of the Veil between Ayndrinor and the negative planes. Morwyn and Divone were able to exert their influence on the world more strongly once again, having been relegated to quiet whisperings in a few people's ears for several centuries prior. They were able to once again send their minions to Ayndrinor at times, opening portals every few months to send waves of acolytes and henchmen forth to the world.
Knowledge in Ayndrinor of The Smiting is vague and niche, with the general public having a general idea of what happened but the true nature of the event only known to the most learned sages and most trusted High Priests. Rumor and legend abound of what the world was like before, and it has become something of a tradition for bards and writers to imagine more extravagant and advanced possibilities, leading to a popular genre that resembles Science Fiction, though the stories mainly take place in Ayndrinor's past. The gods' reasons for undertaking such a monumental and devastating action are not known to any mortal, however, and they patiently sit and watch hoping they will never have to undertake such an intervention again.