The Fold, also known as The Great Merger, was a metamagical event that led to the devastation of the entire multi-verse, as well as the creation of the current "soft-reality" we all currently live in.
Effects Of The Fold
The primary result of The Fold was to collapse every multi-verse into a single liminal space. Every reality that previously inhabited its own distinct meta-plane instantly and irreversibly pressed into the same physical space. The effects of this were disastrous for most, as the competing rules of each reality fought for dominance, backed by their respective powers. The Fold Wars raged for an unknowable amount of time, as time was itself one of the many battlefields this conflict took place over. The ultimate victors of the war set the current rules of reality, which thankfully allowed the continued existence of life, albeit in a radically different and some would say more interesting way.
One obvious effect of The Fold was the reformation of the planet into The Flattened Sphere and the total destruction of over 80% of its total mass. With very few exceptions, every major population center was wiped out during The Fold, and almost every living being was removed from the observable reality plane. Many would return, of course, thanks to The Dread Lich Draxxus Bombastus's app, Bonr. The geographic space once held entirely by the former planet earth is now shared by many planets and reality spaces, such as The Unconquerable Darkness, Shadowbane, Surfhalla, and Kangrank-Nu.
The introduction of magic and magical power is another overt change in reality - whereas the world pre-Fold was a mundane reality lacking in any magic besides the unerring creative spirit of humanity, the post-Fold reality is abundant in every shape and form of magic imaginable.
The Fold also enabled the existence of Powerful Beings, for which we are all eternally grateful. Many consider the creation of these powerful creatures, and the commerce they enable to be such a massive boon to the world that they completely outweigh the overall damaged caused by the event.
The Fold occurred roughly 300 standard Earth Years ago. World history before The Fold is almost impossible to piece together, given the temporal effects of The Fold, and the first hundred years after The Fold are equally hard to piece together. The Fold occured in all parts of the universe at once, and an inciting incident, if one exists, has not been found.
The world flattened, that's the best way I can describe it. The sky sunk down, and the buildings around me squashed down, and so did I. My vision merged with the floor and I could see millions of layers of the world, wafer thin, all smashing together. The noise was deafening, the sound of an entire planet being smashed in an industrial crusher times a billion. I would catch glimpses of other worlds, but just that, glimpses. I don't remember much else but I must have been there for a long time. - Kaltegar, The Ancient One, only living survivor with memory of Pre-Fold society
Post Fold Reality
The new reality we live in is often referred to as "soft". In the pre-Fold world, a lack of magic and a strong physical ruleset made for a more "grounded" world, with little in the way of unexplainable phenomena and dramatic changes in the fundamental rules of reality. The collapsing of all realities has made our universe significantly more vulnerable to these sorts of events. The presence of magic, for example, is enabled by the weakened reality. There also exists certain geographical locations where the rules of reality are fundamentally different than they are anywhere else. Scholars are split on if this is the new normal state of reality, or if reality will eventually assert a single stable set of rules given enough time.