Dealing with the Demon

Bill decides he liked it in Dipper's body,after a year they come back to Gravity Falls for good,but Bill had other plans & ends up killing Soos,Stan and Mabel.But he left Opal. Her only hope is to cope with the insane life of being able to see a ghost & living with a demon. Allie sees that she needs to open the Mystery Shack.But it looks like she's going to need some serious help.