The Shattering is a state of dimensional fracture generated by extreme stress on the established order of reality. Due to a multitude of factors, it caused the demolition of the previous 'universe'.
After the events of the Lovecraftian war and the presence of powerful cosmic forces on the Earth for so long, the normal behavior and resistance of reality was put under a serious level of intense stress. This situation continued to escalate until finally reaching a pinnacle
According to what little can be understood is that the universe effectively "cracks" not unlike a sheet of glass. Slowly the entire dimension begins breaking away, revealing an unknown mass behind it that devoured the universe.
Due to this effect, what remains in the universe after it cracked away was either annhilated by the destruction of effectively 'slammed' into another universe. The universe that exists is not a single universe, but the result of the destruction of multiple congruent universes being simultaneously cracked along with combining factors from their own universes. Those that were not demolished entirely found themselves being crushed into a new universe.
Despite the fact that the universe isn't even a year old, historical text, ancient artifacts, and scientific research, have managed to date the universe as being billions of years old, which means that the new reality caused by the merging of so many resulted in the universe created sort of 'filling in the gaps', resulting in creating a world that is both long and old, and yet created in its exact state as of recent. The actual phenomena behind such a massive event isn't understood, and whether this is an effect of the universe itself or the work of some being can never truly be known. It was no different than twenty houses of cards falling and being rebuilt into one house with many cards from many decks in one absolutely coincidental situation that could be run a million times and not once give the exact same result, while others argue this point that some higher power likely just recreated everything and filled in the holes, and that all of the existing individuals from previous universes that survived were likely little more than complete luck. Still, based on the number of individuals that survived, the assumption is less than 0.0006% of all living beings in a single universe manage to survive the event, if even that much.
The most unusual effect of The Shattering came in the form of a sort of dampening effect. In prior universes (this does not hold true for all), magical and superhuman ability were not uncommon, and thus many of these universes had such as a constant in their realms. Furthermore, magical technology, public knowledge of the existence of Fears, and the ability for organizations such as Black Eden to rise as planetary superpoweres were not unheard of. The Shattering seemed to cause an unknown effect which completely resulted in the toning and, in some magical schools of training, total deconstruction of such powers. More than a vast majority of individuals, as a result, now lack the ability to produce any such amazing effects, and less than the number one could count on both hands likely have power that could be considered "dangerous".