I don't know if it can be classified as dark, because it's in a fantasy world and most of the smut is consensual, but I recommend MADK: wonderful art and story, has a cannibalistic theme and monster fucking. The MC is initially very naive but smart enough.
This one is a short one, but the side stories do go pretty dark for such a short series:
https://www.mangago.zone/read-manga/that_sexapparition_mgs/
Starving anonymous
Oh when I say dark I don't mean flat out rape so thanks I'll check this out
Give me something suuuuuper dark. Everything goes. The Only thing I ask is that the mc is smart don't have to be the smartest or the strongest but pls no idiots.
Something dark and psychological