What is the story of the prophecy behind the "Cipher Wheel"? Spoilers for W3. Gravity Falls belongs to Alex Hirsch and Disney, not me.
Zodiac Prophecy
Chapter 1: Covered Wagon
The young native American shaman meditated that afternoon in the sacred cave, where the first shaman had recorded the summoning of the Beast with One Eye. Centuries had passed and that one had not been summoned again. The shamans guarded the cave, to prevent anyone else from learning how to summon the evil one, handing down the secret from one generation to the next.
The whole valley belong to his tribe, but now pale-skinned outsiders were coming through. The Oregon Trail ran through this area. It went up a winding switchback road, down into this valley for water, then out on the other side.
His tribe traded with the outsiders. One man, who called himself a missionary, gave the Shaman a strange book of wisdom. He had tried to learn to read it, after being taught the sounds of the letters. The Shaman could already read the ancient prophecy script, so it was not that hard to pick up.
Deep in his meditative trance, there came a sudden burst of revelation. Two children were in a settler's wagon, coming up the trail. There was something strange about them; they didn't belong there. They were charged with power from moving in time. Time tries to fight such changes, so it seemed they had created a destiny for themselves.
He had to know more. The Shaman focused as he had been trained, and read from their minds.
A boy with a Pine Tree symbol on his hat. He had started all of this.
A young woman, and a torn bag the boy had been trying to get for her for some reason. It was marked with an English word the Shaman could read, "ICE."
A rival boy with a Stitched Heart on his clothing. The Pine Tree boy was trying to keep the woman he liked away from him, and that is how all the time trouble began.
The Shaman began branching out to others whose lives the boy had touched, or would touch someday. The time energy surrounding them allowed him to probe their future.
A journal with a Six-Fingered Hand, and the man who wrote it. He glimpsed the man in the boy's future, seen by the boy with great admiration.
Another scholarly man who had once worked with that writer to build something. The image was fuzzy. The boy knew the scholar, but not as he used to be. He used to wear the things some of the pale-skinned people put in front of their eyes, called Glasses. Unable to see exactly what those Glasses looked like, the Shaman substituted the image of the ones worn by the author.
An old man with a strange Hat Symbol, related to the two children and to the Six-Fingered Hand. His brother was lost and would be returned, at the cost of great danger to the world.
The Shaman turned his mind to the girl. He saw in her mind many colorful warm clothes, but her favorite of these garments had a Shooting Star. He picked that as her symbol.
In Shooting Star's mind he saw a rival girl, her opposite, who had taken from her a pig that she wanted. That was why she was here, because of trying to change that outcome in her favor.
Looking into her future, he saw the image of a garment Shooting Star no longer liked, with something like a Fuzzy Horse on it. While wearing it, she had gone out with a young magic user with the symbol of an Eye in a Star on his clothing, and that relationship had gone sour. In her future, he saw Shooting Star giving the despised clothing to the rival girl, because she didn't care for her enough to make a new one, as she did for many others.
In his mind, the Shaman assigned the symbol of the Fuzzy Horse to this rival, with the symbol facing in the opposite direction to show her as the opposite of Shooting Star.
Then there was a man with a Question Mark on his clothing, who both the young people liked. Something was strange. They had touched his life before when he was very young, leading to him holding the job he now had. How could this be?
Probing the future further, the Shaman found something very troubling. Meddling twice in time had completely set their destiny. It meant they would cross the path of another man who traveled in time. The Shaman sensed this would cause the world great harm. The old prophecy would come to pass: "When gravity falls and earth becomes sky, fear the Beast with just One Eye."
But perhaps, if he gave them a warning prophecy, that damage could be undone. This set of ten people he had seen could do it, charged with the power the two young people had accumulated by moving in time, if they all joined together in the right way. It would cost all of their lives, but they could banish the demon and seal him away from the world.
The Shaman painted on the cave wall a Wheel showing his vision of what they must do.
That night, the yellow demon came in his dreams.