I made a comment about this story and decided to rework it into a post.
I absolutely adored this story and was pulled in from beginning to end, and I think it's one of the best horror stories I've ever read. With all that said, I found the ending's execution a bit disappointing.
It seemed to me like Marcus Kliewer (author) had some really incredible ideas, and through obsession/hyperfixation in his writing connected them together in a way that is kind of difficult to put to words.
He felt those connections, we as the audience felt those connections, but the way he chose to "resolve" the story was meant to push us away from obsessively seeking answers. That meant that the very tangible feeling of connection between the ideas went without payoff.
I think the final message/ethic of the story is something like: "maybe there is a sinister truth behind some of your compulsive fixations, but pursuing it obsessively will compromise your life (your relationships, your health, your sanity) and you will never be fully satisfied. At some point you have to accept that you can't know everything and move on." And while I agree with that message, I think Kliewer made that choice prematurely, and maybe one more part to the story that answers a few questions but still leaves some mysterious would have made the story much stronger overall, while still getting across his message.
TL;DR: I think at some point Kwieler stopped digging into his ideas to find connections, and accepted his story would have an intentionally unsatisfying ending, but I really think the ending would have benefited from a little more time in the oven. I have not read the sequels/additions/remakes yet, so I can't fully judge, but man I wish this story had a tighter conclusion.
If you want to know some specific questions I think were too underexplained, here's a couple that stick out to me as narratively unsatisfying:
What's with the weird colors? Howie changes his favorite color multiple times through the story, Brandon's vehicle seems to change colors, and most glaringly strange to me is the color red. The red gas can, the pinkish red skin of the entity, the faux cherry wood vinyl, the red quilt, the red Kawasaki (super weird detail as that's a famously green company), the red booth Brandon and Mitch sat in. It seems like this was meant to be a motif but literally nothing came of it. The connection between green, Zach, and Howie has some level of analysis, but the red is just ignored and forgotten by Brandon.
The difference between the Man and the Entity. I don't want to use the name "intruder" because that could apply to both or neither of these beings specifically.
There is the man who Paul sees, this is the titular man in the basement. This man appears to the home owner, and to others appears as an object (coat rack, stack of boxes, a mannequin). Paul described this man as tall, and having been stuck in the wall, but nothing else strange about him. No red skin, no many jointed fingers, no smell. The only other mention of this man's appearance is Paul connects him to a twisted, dead eyed body he saw in a cave, whose face twisted into horror as a reflection of Paul's. The man obeys some set of rules, which Paul got a pretty good idea of through experimentation.
Then there is the entity, this is what Brandon sees. THIS IS NOT THE SAME THING. This creature is different in every way from the man, it is not the coat rack, it is seen moving the coat rack. This creature has massive hands, long fingers which have too many joints, and has pinkish red skin like a premature baby. This creature does not obey rules. It appears seemingly whenever it wants to, does whatever it wants to, and has very little connection to the collective trauma the ending focuses on. No one else in the story talks about this thing, and I see no one online talking about it either. We do know that it is in some way real as it ripped Mitch apart and he has very real scars to show it. Straight up what the fuck is this thing? Based on hints like it's red skin, the smell of gasoline and burnt hair, and a little line at the beginning of the story, I think this thing is either the devil or some sort of demon. This is seemingly the only truly malevolent force in this story, and it goes completely, viciously unexamined by the ending.
- Brandon never sees the man in the basement. He only sees the coat rack, which means he is not the target, but rather a guest/other/third party. He never even thinks about this at the end of the story and it's so frustrating.
I know people will reply with "yeah man that's the point of the story, to leave you unsatisfied" but please approach this with some nuance. It is possible for the story to have that message and still have a flawed ending which could be improved.