Should a setting go into the gritty details of the horrible things that happen to show how awful they are? (Torture, brutal violence, rape, etc)? Or does grimdark work better when it's subtle and leaves things to the reader's imagination? When does a dark fantasy setting become just gore porn? And how do you avoid writing said edginess just to make it darker?
IMO it is the job of the author to guide the imagination of the reader, not force the imagination of the reader too specifically. So some details are important, but a blow by blow is seldom necessary. (Sometimes it is...)
This is analogous to sex scenes. If the sex scene is pretty much generic sex with nothing either character did not expect, then we can gloss over the sexual act itself; perhaps show the beginning of it (getting undressed). I feel like the more inexperienced one (or more) participants are, the more important it is to show what they are feeling. The reader is reading to experience the life of some character; and something like the loss of virginity may be an important character development point.
The same rule applies to the reader experience. If I have a story in which my pretty young female character lures criminal politicians (in her opinion) into sexual situations and then murders them during the act of copulation: She may be accustomed to this practice, her victims may be accustomed to having sex, but the reader needs to see at least one of these scenes to truly understand this young woman. I can't gloss over the details of what she is doing or how she kills them. But I don't need to show it every time, perhaps once near the front, and even if there are several such victims; perhaps once more for the one that got away or whose murder gets her identified or captured.
The same is true for violence. I would feel the need to show it, at least once, to drive home the horror for the characters and their feelings about it.
In describing sex, I recommend that even if you are describing details in an extended sex scene (not cutting away) the focus should remain less on physical action and more on character thoughts and emotions. Always skip repetitive actions (they are boring), and do not get "purple": Not every orgasm is the most powerful orgasm ever felt. I don't mean to be clinical, but to be realistic and if possible use as few superlatives as possible.
The same thing goes for violence. Guide the imagination, don't force it. Don't forget the reality of pain; there is a limit to what people can feel before shock shuts it off; and often what looks horrific to us looking at a victim is not being processed by the victim. I was in a car wreck, as a passenger with an idiot driver as a teen (Hi Tim) and he broke both his knees; due to shock he did not even realize this until he found he couldn't stand or walk. A friend of mine, from a different car wreck, was sitting in the front passenger seat when a car T-boned them on that side. She had her right femur broken in two places. she was wearing shorts, and the shattered bone was sticking out of her bleeding flesh. She said she saw that after the wreck and passed out. She remembers the image, but does not remember feeling any pain. Likewise, people have been gunshot or knifed in a fight and not realized it until later.
Sometimes well chosen details are enough to guide the imagination without any horror adjectives. I'd keep them sparse, this is like a partnership with your reader. My approach is to provide matter-of-fact narration about what is happening and what my characters are feeling (without superlatives), and I leave some of the emotion to readers by putting them in the situation and trusting them to empathize with the character being tortured.
It is only gore porn if there is no real story or plot. It is not a real plot, in pornography, that the pizza guy comes and a beautiful girl wants him to bone her. Maybe that happens but it isn't a story. A story needs a hero (or group) with some arc of a problem introduced, pursued, and they conquer it or it conquers them. Make sure you have a real story, characters that struggle with a problem and evolve and change in some way, and in the end come to some resolution.
For what it's worth, because this is a pretty subjective topic, I think the less you show your audience in horror the more terrifying it becomes. You have to set a scene certainly but the more you get people to project their own fears into situations the more people you will scare and the better the fright will be.
Look at how little H.P. Lovecraft actually says about the form of his many terrors, he leaves the reader to fill in a lot of the details and they're all the more horrible for it. So set the scene and let people scare themselves.
From a pure publishing perspective, super explicit depictions will turn a lot of publishers off, nevermind readers.
I have a pretty high tolerance, and usually what I ask is how neccesarry a scene like that really is for the plot and contexr. If its not, I usually lose respect for the author.
Lighter is better. And if you're going there, do so sparingly and for good reason.