In this story both Bill Cipher and Will Cipher exist.
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Mabel and Dipper Pines weren't normal children. They were distant, analytical and perhaps a little cruel at times. They meant well but their state of being had always been sobered by the fact that at a young age they witnessed their mother and father succumb to the temptation of demons. Together they swore that they'd never let a demon destroy them, never let the world they were born in take advantage of them. And thus began to decent into magic. Small things at first, moving objects with their minds, changing small things into other small things, eventually they grew bored with the smaller scale magic and began to extend their knowledge into more interesting and dangerous abilities. Manipulation of space and time, possessing of other people's bodies, summoning demons. This of course caught the attention of the FBI but rather than putting an end to the dangerous art the twins were studying they sought to use it in their favour. Training the Pines into merciless agents who's purpose was to keep the weird world as much of a secret as possible.
To say they were good at their job was an understatement, their hearts already shielded from the horrors they committed, to do what they had been trained to. At the young age of twelve they had already become notorious hunters of the weird that refused to stay hidden. Eradicating the rebellious and on good days rehabilitating those who could be saved. They often stayed in one place for a while, finding and either taming or eradicating the source of the problem. The places they were sent were places that had high concentrations of weird. Which is what drew them to Gravity Falls.