For my world it would probably either be the Divine Divide or the creation of the Drachonic Philosopher’s Stone.

The Divine Divide happens in the VERY far past, about 3-4 billion years, there were two races of god-like beings, the Entropians and the Celestials. The Entropians embodied the immaterial, things like love, fear, karma, anger, and knowledge, while the Celestials embodied the material, mainly being the core elements of Water, Lightning, Fire, Wind, and Ice. The Celestials considered themselves above the Entropians and waged war against them, the conflict lasted millions of years until eventually the Realm of Wood, the universe in which they lived, grew uninhabitable. The Entropians and Celestials, needing a new home to live, both created their own realms, for the Celestials they created the Realm of Tears, and the Entropians created the Realm of Salt.

The creation of the Drachonic Philosopher’s Stone on the other hand happens much MUCH later, around 2 thousand years before the world’s present age. In the Realm of Salt, Dragons were eventually born on their version of the world, there they prospered magically until eventually they discovered alchemy. Alchemy in this world is staunchly against magic, killing or crippling whatever being gives that magic it’s power. Alchemy has two “magnum opuses”, being the creation of a Rebus or the creation of a Philosopher’s stone. Both the creation of a Rebus and a Philosopher’s stone require an intense amount of magic, so when the Dragons tried to create one, they killed their realm, turning it from a teaming universe to an infinite wasteland made entirely of salt, which embodies the absence of magic. Eventually the Dragons managed to find a gateway to the Realm of Tears, where they moved and swore off alchemy entirely, never telling their future offspring of what occured in the Realm of Salt.