Transcendence
An event that shook the world, the Transcendence is an event that took place after the Twins thwarted Bill Ciphers plan to merge the Mindscape with the physical plane. This allowed for the supernatural occurrence that only took place in Gravity Falls to spread throughout the Earth and alter the course of history.
Bill Ciphers Plan
The Mindscape and the physical plane have always been kept separate from each other, interacting very rarely, if they were to interact at all. Bill's plan was to merge the two together and unleash chaos upon the world simply for his own amusement.
His plan had taken thousands of years to put into place, waiting for the universe to align itself properly, altering civilization to go the right direction he needed it to be for his spell to work. The last piece of his puzzle being Gravity Falls, where he had done a minor experiment 30 years ago, leaving a sliver of the Mindscape open and a barrier around the town to hold it all in.
Mabel and Dipper managed to pull together a team of people (their friends and some others from the town who knew of supernatural occurrences) to stop him. They manage to follow the clues and manage to just barely stop Bill. Putting his thousands of years of work to waste, however, the barrier keeping the small sliver of the Mindscape within Gravity Falls was shattered, causing the supernatural to spread to the rest of the world.
The Aftermath and Dippers Predicament
Immediately after Bills plan was thwarted, he was barely clinging to life after receiving the full force of the backlash from his broken spell. Almost completely out of energy, he made a huge gamble on possessing Dipper once more to survive. His plan failed, causing his mind to vanish and his remaining energy was merged with Dipper.
Having merged with Bill Ciphers energy, Dipper was no longer a human. Now existing within the Mindscape, invisible to all but Mabel, he himself had become a demon.
Post-Transcendence World
The Transcendence itself happened around the world instantaneously. However, mankind took much longer to adjust to the new environment. It takes a few years for the worldwide panic to subside and for the supernatural to be more accepted and seen as a part of every day life.
