Imaginary Friend by Stephen Chbosky

A boy returns from the woods with a mission from an invisible friend that will decide the fate of his town—and his soul.

Kate Reese and her son, Christopher, flee to Mill Grove, Pennsylvania, hoping to outrun a violent past. Their peace ends when Christopher disappears into the woods for six days. He returns unharmed but changed, guided by a voice only he can hear. This "imaginary friend" has a single, urgent command: build a treehouse in the woods before Christmas, or the town will fall to an ancient, creeping evil. Imaginary Friend is a sprawling supernatural epic that pits a mother’s devotion against a literal nightmare world.