It's about a girl whose parents believe she is possessed. Because they recently lost their jobs, they accept to having the exorcism be recorded and made into a reality, called The Possession. The story is told through the perspective of her 8-year-old sister. I don't want to give spoilers, so I'll just say that this turns out to be not your classical exorcism story, it has a lot of room for social commentary, and that the ending is very unnerving.

Like I saw someone describe it in Goodreads, it's not that gory or overly scary. It's more like it leaves you with a sense of urgency and inevitability, as the family keeps going forward with the reality, and unease after you discover what happens following the exorcism.