BottomlessPitTale (Mabel and Dipper fall into the Underground)

Cover art is NOT MINE! It was one of my computer backgrounds! I wish I knew whose it was! If you can tell me whose it was please do in the comments!

I was never quite satisfied with the Gravity Falls/Undertale crossovers I read, so I decided I ought to write one I was satisfied with. I'll take my favorite elements from the different versions, credit them, and add them to my own ideas. The dumb title is just to make it unique and I may change it later.
The beginning especially is a megadetailed version of a story called Twintale on Archive of Our Own. That story stopped after Mabel and Dipper left the ruins, but it contained lots of things that I thought should be no-brainers in a GF/UT crossover, or at least ideas that would be good in a crossover that don't seem very popular such as:
*The twins get there via the Bottomless Pit.
*Bill follows them and teams up with Flowey.
*Mabel has a green soul and Dipper has a purple soul.
*This happens as an alternate turn of events toward the end of the show, when the twins already have significant, established relationships with all the characters and the supernatural, not to mention it happens in a very emotionally charged episode and scene.
*They fall INSTEAD of Frisk (and one of the other children, presumably).
*Bill has a very specific reason for following them like this, something he wants (the Rift).