I've only seen the first Paranormal Activity so I'm reviewing this as a standalone movie, even though it's actually a spin off or whatever. It's the same premise though - found-footage, teenagers, ghosts, demons, and what have you.
The first 30 or so minutes I was invested in the movie. I really was. I liked the characters, I thought the filmmakers were doing something interesting and fresh, there are some comedic bits thrown around, it was just enjoyable to watch. Then it turns into cliché city. Easy jump scares, stupid mythology that isn't even explained and makes no sense, people running around from room to room with no suspense. It's like halfway through the movie the filmmakers were like "oh crap, this has to be a Paranormal Activity movie. Let's just make a bunch of supernatural crap happen to these characters we developed and wrap this up as quickly as possible". And that's how the entire third act feels: rushed and lazy. It basically went from being an intriguing murder mystery to a generic possession movie.
I won't say it's terrible because I didn't hate myself watching it. It just got really uninteresting really fast. Near the end I was just waiting for it to be over because you know there's not going to be any explanation or any depth to the story at that point. You just want these stupid jump scares to stop so you can go about your life not watching this movie. There are worse found-footage movies out there, but The Marked Ones is definitely one you can skip over.