The day a reluctant Dipper Pines made a hasty deal with the triangular dream demon was the day the young boy died.
Don't worry, he was still alive in a way, and so was his body. But still, things evolved into a horrible situation, and, in a way, it's slightly difficult to explain exactly what happened. You see, Bill Cipher, the demon whom has been spoken of previously, offered Dipper a chance to unravel all the secrets almost torturing the twelve year old by giving him the eight character password to a laptop the boy had found, one that had previously been owned by the mysterious author that had written the journal now belonging to the child as well, containing information about various conspiracies and monsters hidden throughout Gravity Falls, the sleepy little town he currently resided in with his uncle and sister. Dipper knew the computer would be a key component of finding out everything there was to know about the mysterious place he so craved to uncover, desperate to know what was really going on in an area where everyone denied every piece of evidence strange things were in fact going on, but he was held back by only the password that denied him entrance to the facts and information he needed.
Of course, Bill knew exactly what the password was, as he was a being with knowledge of practically everything, specifically an expert on things to do with the town Dipper was interested in. It was a crucial place to what he knew would eventually happen in time, but all the mysteries would have to never be uncloaked by anyone for such a timeline to happen. With that said, it should simply be stated that the young Pines child was in his way. So the triangular demon visited Gravity Falls with a specific reason- to make a deal with the person getting the most in his way.
And you probably know by now that it had to have been accepted. Bill twisted his words so that it sounded like he only wanted a small puppet Mabel, Dipper's sister, had been working on in exchange for the various eight letters and numbers the boy needed to crack the case of everything mysterious and unexplainable. At first, he actually did turn the offer down, remembering that only a month or so ago he had needed to literally enter his uncle's mindscape to drive out the same paranormal being now trying to possibly be kind to him, but after careful persuasion from Cipher, whom reminded him that his twin sister had never done anything in return for him, and with the fact that the laptop was actually set to erase its data at any given second, the deal went through, and he was then allowed to get his 'puppet'.
What Dipper didn't know was that by puppet, he had meant his body.
Before he could really say much else save for asking which puppet Cipher would pick, as there were many puppets available to choose from, Bill had responded simply.
"Hmm, let's see...eenie, meenie, minie, you."
Without another word, he ripped the soul right out of Dipper's physical being, going in himself and possessing it.
There you have it- the confusing and almost disturbing story of how the youthful, conspiracy-believing Dipper Pines became a practically nonexistent being, and how his body became controlled by a demonic lunatic.
But the story doesn't end there. There's still so much more to tell.
After Dipper's physical self was taken over, Bill destroyed the laptop and was about to do the same to the boy's precious journal when the Pines family headed to a local theatre to see a rather elaborate puppet show being put on by Mabel, with Bill-Dipper(or as Mabel would later call him, 'Bipper') coming along, realizing the book he was looking for was being used as a prop in the show. The ghost of the real family member was soon left behind, since nobody could see or hear him besides the very thing that took him over. In fact, the car everyone had gotten into passed right through him, leaving him to begin to fear beyond belief what was going to happen.
It wasn't very long before they all arrived, the ghost of what was once the kid now sitting in the front row of the theatre trailing behind, and the possessed, so-called puppet seized the opportunity to ask his 'beloved' sister for the last-minute role of the reverend as soon as he learned that the journal would be used as a prop for a wedding scene within the show. Excitedly, she accepted his offer, and the show began.
Most of you know as well what happened here, but let me stop you right there. I must tell you, warn you, before you read on, things went very differently than what has been told before.
These next moments are where everything changed.