This might as well be the Post-Release Thread for this movie, so I might as well give out some spoilers.
But first, I'll give a short non-spoiler take on it in case there's actually anyone reading this who doesn't want anything ruined.
The movie sucks... I mean, it stinks. Let me just say that I'd rather try out autoerotic asphyxiation than have watched that film! It's not scary or terrifying at all.
As for what's been stated already, yes the movie has no ending and instead tells you to look at a website to look for the story to be finished.
Problem is... the website does not finish the story at all! What a double middle finger to everyone. And Jeebus, what a website it is, but I'll explain why it's so awful a little later.
Now, onto spoiling things:
As the trailers have stated, a young woman Isabella (who at least isn't bad to look at) goes to Rome as her mom is at a psych ward there after she killed three members of the clergy who tried to exorcise her. She brings along a documentary filmmaker and his cameras to document it, because... well, why not? This results in the print usually looking pretty nice for a docu., although of course there are times where you see shots that are impossible to have been captured unless this was a movie; way to keep up the illusion. Also, of course you see everything, which is rather implausible for a found footage movie.
Before Isabella meets up with her mom, she meets a pair of priests at the Vatican who are "rebels" and do exorcisms outside of the realm of the Church. Yes, really. They actually bring up an interesting idea about what is being possessed and what is mental illness, but it's tossed away as you get a lot of talk from Isabella, the priests, and even the documentary dude; sometimes it's them talking one on one to the camera and it's just yawn. There's also a lot of griping about the bureaucracy of the Church; oh, how original and controversial. The first meeting with mom doesn't go well, so after Isabella and the cameras are brought along to an exorcism of a teenage girl (is this a good idea, logically?), they exorcise mom and it seemed to go well... except that (as blatantly hinted at early on) demons can possess someone else after they leave the body. So, you see one of the priests get possessed and he actually almost drowns a baby at baptism. That (along with the very beginning of the movie where you see fake news footage of the mom getting arrested) was the only part that inspired any sort of goosebumps being raised. Still, I wasn't pissing my pants due to fright.
Well, long story short (actually, to be honest, this story wasn't long at all, but you get my point) the priest blows his brains out, so of course Isabella gets possessed. Who could have predicted? She causes havoc at the hospital so she is dragged away with the other priest and the filmmaker drives them elsewhere. Somehow, the filmmaker apparently got possessed somehow in the chaos so he deliberately crashes the car and... the movie stops rather than ends, and you get the note to check out this website for further information on the case... which doesn't advance the story at all.
Fuck You, Movie!
It's just not a scary or terrifying film. It's just dull. Not even the exorcism scenes get the heart rate up. It's laughably pathetic compared to what you see in The Exorcist, for example. And the rites that they give to cast the evil demon from the body is not believable at all, again compared to the William Friedkin classic, not to mention what you can watch on YouTube of alleged exorcisms. You just can't believe the boring story they're trying to tell you. You shouldn't even watch it on DVD/on demand/whatever once it becomes available there.
Then again, this movie was directed by the guy whose only other movie was Stay Alive, an infamous movie in horror circles as it's about a horror videogame where people in the game end up dying in real life and it involves Elizabeth Bathory (!) and somehow she apparently released that videogame in order to find victims to kill (!!)... with that ridiculous horseshit there should be no surprise this movie turned out to be rancidly bad.
As for that website I mentioned, it does not further the story at all. Instead, it's made to look like an incredibly amateur thing (why would you do this?) which has some videos of Isabella as a kid but otherwise it's random shit where Wikipedia links are brought up and they did a YouTube search of "exorcisms" and embedded the first few results they found. The website has 18 different pages so I clicked through them all waiting to see what happened to our three remaining protagonists, only to get jack shit; instead I got misspellings and repeated YouTube videos. Again, why would you do this? Most appalling of all is that on one page the documentary dude's background is explained as he previously directed a movie set at an old mental hospital, and it's given a name. A LITTLE BELOW THAT ON THE SAME PAGE you see a fake poster for said movie and a different name is given for the hospital it's filmed at!
Again, Fuck You, The Devil Inside!