Demon Falls (Demon Road/Gravity Falls crossover)

Sky Template is mostly demon. After escaping from her family, she now lives life on the run.
The Shining Demon gave some of his essence to the newborn to see what would happen.
Because of that, Sky has been able to Shift since 7 years of age and has a few new abilities that not even her family knows of.
She has hidden these abilities by not letting her family see her in her demon form, though she does not know how much longer she can keep it up.

Last summer, the Pines family and their friends have just defeated Bill Cipher and now they want a break from anything weirder than usual at Gravity Falls. Unfortunately, the multiversal transportation machine (the portal) was built between the two summers without anyone knowing who did it.
Something turns the portal on before they could dismantle it again, a girl comes out and practically screams at them to turn it off.

What have they gotten themselves into?