The Winchester never thought they would have to go to Gravity Falls, the place where The Demon of Dreams, Nightmares and Insanity, 2nd Only to Lucifer was locked up.

The Pines thought that all the evil supernatural would be over until they came back the next summer to find creatures killing people and strangely all their weaknesses matches Dippers favourite book series;

Takes Place

Supernatural- After Season 11

-No annoying brat Lucifer

-Cause Im up to season 12 when writing this

-Isn't going to follow season 12

Gravity Falls- A year after their initial adventure

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The Dome that surrounds Gravity Falls is a powerful Devil's trap, keeping all the creatures (Demons, vampires etc) out and Bill Cipher in. The type of devil's trap to use is called a Cipher lock. It was made with full intention to trap Bill inside and keep everything else outside.

The Natives found the zodiac as a way to keep Bill trapped. After Samuel Colt found the cave drawings and the zodiac he teamed up with Rowena to create a more powerful spell that couldn't be broken with Bill being summed. The Spell also had the side effect of powering down Bill and making sure that he couldn't get more powerful.

When Ford summoned Bill over 30 years ago and started building the portal it gave Bill the means to start Weirdmageddon which at enough power would have destroyed the devil's trap completely.

If Weirdmageddon went all the way he would have become an ally to Lucifer in the Apocalypse and his weirdness bubbles would be spread all over the Earth only much more sinister and violent. Millions upon millions of Thrones would be made out of people's burning flesh trapped in stone

Even though Weirdmageddon didn't break The Cipher Lock it cracked it letting all the creatures (minus demons) get in, it wasn't powerful enough to let Bill out but now with Creatures going inside of The Cipher Lock they won't be able to get out and they will go insane.

Will include

-Hinted Destiel/Destiel

-Ford at the side antagonist

-Sorry But No Billdip

-Villain Bill