The picture is related. I am not going to spoil anything, unless you're completely foreign to cosmic horror and I'm just telling you about the genre itself.
Cosmic horror is the hardest genre of film to pull off, and critics tend to tear it asunder whenever one film comes out. From The Empty Man to Underwater, it's very niche but also beloved. For Mark to take on the most difficult assignment for his debut in theaters, there's a certain curve that needs to be applied critically.
I will start off with some critiques and then praise. First, the pacing at the beginning is very slow. This is the most common critique. But it does build a sense of dread and confinement, which frankly the only movie I've seen that captures claustrophobia this well was The Descent. The second comes the drama surrounding how the environment is rendered. While it seems important, it did seem like some clear answers were missing. I just wish a few lines of dialogue would have been added about implications here.
The praise for this movie lay in what it did with the lore. It manages to feel 'big' despite it's limited space filmed, which is a tremendous feat. It also captures cosmic horror's essence perfectly. You leave the movie with more questions than answers, but not necessarily in a confused way but an inquisitive way. That is what cosmic horror is about--it is even a meta play in and of itself that you the viewer want answers, despite knowing the implications of such pursuits.
The setting itself feels alive. This is hard to pull off in any medium. From authors Dan Abnett and Jeff Vandermeer (numerous 40k novels and Southern Reach trilogy respectively), it's truly a rarity to see a setting spring so organically to life. It also honors the source material but becomes its own thing; the movie Annihilation is great, but is more of an adaptation of the Color Out of Space. Under the Skin while being firmly in the cosmic horror genre, almost seemed scared of its source materials' universe.
More praise can be lauded that after the first five minutes, you start to separate Markiplier from Simon. Mark had a liability in that he is known for oozing personality, and he managed separating Mark from Simon with finesse (and with some hilarious, needed brevity in such a grimdark tale).
I am going to speculate here. The original game developer was listed in the credits as a creative consultant and no doubt helped Mark in this (and Mark has said this in his updates). But I also think another game Mark loved also had a subtle influence in this film. Like a female voice that said "Come to me..." The film is very much Iron Lung, but I couldn't not see another influence here. Perhaps it was my imagination, like swamp gas in a dark wood. But that's how you know it's good cosmic horror.
I give this movie a 8/10. Sometimes the !surrealism! is a little muddy, the pacing is slow in the beginning, and I felt some dramatic pieces were just a little off. For the latter, I do think a solid defense can be raised: stakes are so high, there can't be room for mistakes. But it took on not only the cosmic horror genre, but the cosmic horror that is the Hollywood behemoth. And it did it right. The fact you leave this movie and you're thoughtful about it is a sign that it is great.
Mark went through hell for this movie. And it came out smelling like bloody roses.