You can write in detail about "dark" topics, you just have to choose the correct details. It's possible to have a girl get swept up by tentacles. Instead of drawing out what happens once she gets dragged around that corner, you can instead highlight her confusion and fear as she desperately clings to anything that is available to her. You can write a torture scene in the same vein, having a torturer explain what's going to happen to the victim if they don't give up the safe's password. I actually just wrote a scene a week ago where a character explains to someone she captured what's going to happen when they fire a cannonball into their chest. There's no gore, she never fires the cannon, and the reader knows exactly how destructive it would have been. Hell, maybe you don't mind erotica as long as it isn't X-rated. It's possible to be sensual and intimate without "sticking it in." Characters can kiss, hug, and touch with their clothes on. There are also layers of clothing that can be removed. people don't have to *BING* appear naked before feelings of intimacy can be felt. I always laugh when people say "I can't feel the intimacy of these characters when you fade to black." 

Of course, you're always going to run into one-upmanship. "Oh your character lost their finger? Well my character is going to lose their whole hand!" It's hard to regulate this type of behavior, which is why establishing rules in RPs is so important.

Problem with writing dark topics is that there's a fetish for everything. Getting enveloped by tentacles? Obviously. Being consumed? Nothing new. Experiencing an event that completely breaks your psych and renders you an emotionless rag doll? Check, and double check. I tend to avoid too much use of "mainstream" dark when I can for this reason. Your horror roleplay can quickly turn into a sadist sex fantasy if you're not careful. That's not the kind of excitement I was looking for, friends.