The Dangerous Forest Monster

Once Upon A Time...
So after adventuring with the twins for a while, some demon dude realises you have a thing he's been searching for, so he sets up a deal, charming as ever, you hand over that thing in exchange for another thing.
No but seriously, if I were to be specific here, there would be spoilers, so just try to read it and you'll find out what thing 1 and thing 2 are xD
Pros: Dipper and Mabel Pines!
Like, it's a Reader-Insert, so you know, you're in it!!
Cons: That arse Johnny found his way in >_>
And I swear I don't know who broke that wall...
Enjoy!~ ( *｀ω')ﾉ