The Great Crash, or Tari Shock, is an event that had taken place in the Gamindustri of the Ultra Dimension thousands of years ago. In the past, the concept of nations did not exist and CPUs were just the stuff of legend. War was endless and the world was chaotic. After Rei Ryghts had accidentally eaten a CPU Memory and became a goddess, she used her powers to form the nation known as Tari which ruled over all of Gamindustri.
In the beginning, Rei attempted to lead the people of Tari to the best of her abilities, believing it was destiny that she got such powers. Eventually, Rei became extremely tyrannical towards the people of her nation which lead to them rising against her , starting yet another war. She fought against her own people which lead her to realize that a CPUs power comes from the faith of her people. Rei lost her powers and her nation collapsed in a matter of minutes.
After this, the people's hearts grew more uncertain and they didn't know what to do about anything and there was a huge recession in Gamindustri. Rei had nearly lost all of her powers as a goddess and could do no more but think about where she went wrong, being unable to die nor age. She started to believe that the problem lied within all CPUs and thus created the Seven Sages.