The Penumbra collection is composed of two games (Penumbra: Overture & Penumbra: Black Plague) and an expansion (Penumbra: Requiem). The first game starts scary, but outstays its welcome almost immediately. The game likes to hide pieces to its puzzles across several rooms and several load zones, meaning completing puzzles takes way longer, but only because it takes longer to complete the pixel hunt requires to even start the puzzle. The puzzles also have an issue where they like to kill you for getting them wrong, which cheapens death to the point of not being scary. The game then swiftly stops being scary at all once you realize you can kill the monsters and they never get replaced after you do so. The second game fixes this issue by removing combat- one awkward boss fight aside- but keeps the issue of having to run across twenty billion load screens just to solve one puzzle and death is cheapened by failing said puzzles constantly killing you. It's an improvement on Overture, but still very jank overall. Requiem is awful. I'm sorry, I know we're supposed to be constructive, but Requiem is an absolute slog. It's all puzzles, and there's fewer load times now, but the rooms are so massive that minimizing loads fails to correct the issue. The game also kills you a lot less for failing them, but that doesn't matter, because there's basically no atomsphere. Just play the first two and forget this one exists. It's worth playing these games for novelty's sake, but they don't hold up to something like Amnesia. Arguably, they don't even hold up against White Day, which predates it by several years. Perhaps, just for that novelty, it's worth 10 bucks.
You've heard plenty about its story- how it's "existential" (cringe)- and while it is one of the few examples of "existential" (cringe 2: the recringening) horror done well, it's also buried under layers of tonal whiplash. You can literally go from reading someone's dying memories to having your support character get snarky at you within the same ten minutes. The game has monster encounters basically just because Amnesia had them, but they don't work here because the monsters are nonsense puzzles. There's one monster in particular who's basically a game of "red light, green light," and the game completely fails to convey you're supposed to do this. Not to mention they stick out so hard against the "existential" (cringe 3: return of the cringe) storyline that the whole monster subplot (yes, the main threat during gameplay is an element from a subplot) is resolved in what the game even admits was a detour. There's good to be had here, but it's buried under layers of mediocrity. If you have to play this, don't buy it full price. For the love of Satan, don't buy it full price. Wait for a sale or something.