It's getting a bit difficult to find good dark tuned novels these days, that or i am not looking carefully. I know about a couple, like Torture Princess or Elvenblood, but this first one is still on early translation by yenpress, and the second one is on indefinite hiatus for years. Goblin Slayer may be the best choice i can give you for now. Also Grimgar, is not dark tuned most of the time, but when it does it really can get dark and bizarre pretty quickly, especially if we have Haruhiro's POV. There is also To deprive a deprived person, but it's transaltion is getting worse and worse, and everything seems just confusing now, even more with more and more characters that i can keep track off who all seem very important for some reason getting added in the span of 1-3 chapters.
So yeah, Grimgar and Goblin Slayer are the best i can reccomend you for now.
