I Trapped the Devil (2019)
The plot centers around four characters, and one of them spends more than 99% of the movie off-screen. There are also a pair of cops who show up at the start and end, but they're fairly inconsequential.
The three named, significant characters are Matt, Karen, and Steve. Karen is married to Matt, who's Steve's brother, and the couple show up out of the blue at Steve's house on Christmas Eve, expecting him to be happy to see them. There's been some sort of falling out or something, and everyone has secrets.
At least, I think they have secrets. None of that really comes up or gets explained: we're just kind of told there's bad blood, and that Matt wants to make amends. Only Steve doesn't want them there, nor is he receptive to their questions. After a tense dinner, he finally pulls his brother aside and reveals the reason he's been so aloof.
It's... it's in the title. He's got the devil, who's taken on human form, locked behind a door in his basement. The devil, Steve explains, is responsible for some (but not all) of the evil things people do, so containing him will prevent a great deal of suffering. Or at least he has someone locked behind that door - Matt is naturally skeptical, as is Karen when she finds out.
Also, there's a loaded foreshadow. I mean a loaded Chekhov. A gun. There's a loaded...
Never mind. The characters interact with each other and the voice in the basement, who insists he's done nothing wrong and begs to be released. Karen goes down alone and almost opens the door but stops at the last minute. Then the voice starts referring to her by name, laughing, and just kind of acting devilish.
In case anyone out there really thought a horror movie was actually going to decline to put the literal prince of darkness behind those locks. But soon after Steve goes kind of nuts and almost shoots Matt and Karen due to hallucinations. This is all established in advance - we're told the devil can get into your head and all that, but... yeah.
He comes to his senses and hands over the weapon, then Matt decides to let the guy out of the basement. Karen, now believing it's actually the devil, tries to explain that she felt something while she was down there, but Matt doesn't listen: he's kind of losing it himself. I'm a little unclear why Karen doesn't mention that the voice laughed at her maniacally and seemed to know what she was doing, as well as details about her. Also, no one brings up that the devil's making images appear in the static in a television in the living room.
But I guess everyone's just going crazy. Again, this was established as a possibility, so it's not technically a cheat. It is, however, unsatisfying, as the resolution comes from characters deteriorating more from their proximity to the devil than from their relationships. It's a little ambiguous, since they're definitely alluding to shared history, but none of it's explained or all that developed.
Matt tries to release the devil, and Steve stabs him. Then Karen shoots Steve. Whether she's taking revenge for her husband or just driven mad isn't spelled out, so read into that what you will. Then the cops show up, Karen shoots one, and the last frees the devil, who leaves in the form of a little girl.
I kept wanting the characters to be more developed than they were, and I wanted some sort of punch to the ending. I spent the entire movie trying to figure out how they'd handle what was behind the door ("child" was definitely on my list of possibilities).
I almost think they should have gone with having the room be empty. Sure, it's a cliche, but so is anything they could have gone with, and it would probably have been the best cliche available.
Like I said at the start: the script could have used some work. But I also said it's atmospheric. The holiday decorations, coupled with some creepy lighting and set decoration make for a tense setting. And the sequences where characters interact with the devil are well handled. Lucifer's disembodied voice is easily the best character in the movie, which is certainly what you want. Steve's hallucination is also impressively creepy at a key moment, which comes close to actually selling the whole "he just loses his mind" development.
Ultimately, this is a fun movie which, sadly, wastes a lot of potential. A script with better realized human characters could have delivered something really special, but even as it is, it's still a well made, effective piece of genre entertainment. I can't quite give it an unconditional recommendation, but it's worth checking out if you love horror.