Transcendence

The Transcendence refers to an instantaneous, worldwide event that took place when the twins were twelve. Bill Cipher's attempt to fuse the Mindscape/Nightmare Realm with the physical world did not have the effect he intended, and instead brought the supernatural oddities of Gravity Falls all over the globe, revealing magic where it had been hidden and creating magic where there hadn't been any.

Origin

For thousands of years before the Transcendence, the dream demon Bill Cipher had been planning and setting up a massive and complex spell. This spell was designed to merge together the separate Mindscape and physical plane into one. This would cause widespread chaos and tragedy throughout the world and bring human civilization to its end. Not being inherently good or evil, Bill Cipher simply wanted to have fun and laugh at the chaos his spell would bring to the world.

Being one of the most powerful beings in the universe, he altered human history and steered the civilizations of earth in the right direction to make reality the precise way he needed it to be for his spell to work, and then waited for the right moment when all of the universe had aligned to unleash its power. After thousands of years of work, he did a small test run 30 years before the Transcendence, opening up a small sliver of the Mindscape and containing it within the small town of Gravity Falls.

Dipper and Mabel Pines had followed the clues left behind and formed a small team of friends and others who knew of the supernatural occurrences in town to try and stop his plan from succeeding. In the end, they just barely managed to defeat him and saved the world from the disaster his spell would have caused. However, the spell's backlash went not only to Bill but also to the barrier surrounding the town and keeping in the sliver of Mindscape. With the barrier shattered, the whole world was plunged into confusion and upheaval as supernatural occurrences appeared around the globe.

The Aftermath and Dipper's Predicament

Immediately after the spell was stopped, most of the backlash of its power went directly to Bill, causing him to lose much of his power, and left him just barely clinging to life. In order to survive, he made a huge gamble on his part, attempting to forcefully possess Dipper's body in hopes of a physical form being able to hold him together until he could regain his strength. A demon as powerful as him could do it without a deal, but in his weakened state there was a high risk that we would fail and die in the process.

While he had the bonus of inhabiting Dipper's body before, he still was not strong enough. Dipper fought against the possession with all of his might and in the end, Bill could not win. He dissipated and his energy merged with Dipper, effectively pulling him into the Mindscape and making him a demon as well.

Post-Transcendence Civilization

The Transcendence allowed for a large variety of creatures that were previously incapable of interacting with the physical wold, or else posing only a very limited in doing so, a greater presence. While society initially reeled from the supernatural occurrences spontaneously popping up all over the world, Gravity Falls remained largely unaffected, being pretty used to such things already.

Eventually, the rest of the world develops a basic idea of what happened during the Transcendence, and come to recognize Gravity Falls as its epicenter. School systems around the country begin to implement new courses designed to inform children about the supernatural in general, as well as the dangers such things potentially pose. While these programs are notoriously uninformed at their inception, they do become reputable as time passes.

New political parties are formed in response to this new influx of supernatural exposure, such as the Pro-Nats (Pro Naturals), who tend to be against equal rights or affirmative action for beings of magical origin.

A brand new calendar system eventually comes into fashion, largely replacing the traditional 'BC AD' with AT (after Transcendence) and BT (before Transcendence), with the year 2012 being 0 AT.