Ragnarok

In 18,000 BCE, the ancient race of the Guardians of the Universe implemented a protocol to contain the race of ancient warriors known as Asgardians. They attacked key components of the 'neural network' that made up the Demiurge. The result devastated Yggdrasil, the World Tree. The New Gods on the most distant edges of the tree became isolated and unable to commune with their followers. Those lands and dimensions most dependent on magic to sustain themselves were wracked by natural disasters. Asgard's magical defense failed her and their weapons were virtually useless, allowing the mighty Surtr to invade destroy the Golden City. The Nine Realms in particular were torn apart by war, and the population of the Aesir reduced to less than a hundred.

On Earth, the mighty nations of Atlantis were devastated. They were entirely dependent on magic in every aspect of their lives. Wells produced no water, crops withered, and entire cities crumbled. The mountains fell and the seas rose, and the world entered a brutal ice age called the Hyborean Era. Only the very mightiest of sorcerers were able to survive with the meager fraction of power remaining to them.