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I don't know what do you mean exactly by "dark", but there is a VN in my mind might match. You can take a look at: Suteki na Kanojo no Tsukurikata: https://vndb.org/v29404 It's not the best VN if we look at the overall rating, but It is quite interesting. If you're looking for a "sickening dark psychological" story and atmosphere, then give it a shot. Don't raise your expectations though.

Well, if you're looking for some VN with some depressing contents, then perhaps you can try Fata Morgana if you still didn't play it yet. Other than that, perhaps you can try Tsui no Sora Remake, Dead End Aegis, or Gore Screaming Show, although note that those three VNs have disturbing contents if that's bother you. I don't know if you're already play those or not, but regardless I hope my recommendations will be helpful to you.