toucanbuzz
No rule is inviolate
If your players know the setting, or if you know it so well you can make it come to life for them, that's always a great idea.I'm toying with the idea of constructing a campaign around something with the feel of the Witcher...
It's not the rules. D&D avoids too much dark in official modules because it wants to appeal generically to all audiences. Modules are like coloring books to me, a framework for me to mold them. My Curse of Strahd was far darker, for example, than the module. The rules can work just fine. What you'll want to do is setup limits on what races & classes/subtypes are allowed, and even changes to magic (e.g. limiting cantrips to a # per day).Of course, given that most D&D modules are kind of "heroic" (and less depraved), most don't really fit this kind of mindset. I'd like to stick with 5th edition...
If all your players know the Witcher setting, they might work with you on eliminating certain classes from the campaign, such as bards (there are bards, but only in the musical sense, probably more rogues), clerics, druids, paladins, sorcerers, and wizards (make all casters warlocks, mechanically, for example). From there, you can limit races down to the setting, knowing the rich history of the conflict between the human world and scoia'tael.
You can slow character advancement, but otherwise, it's been the subject of much debate about the power creep of D&D into relative superheroes by the time 12th level or so rolls around. Personally, I'd slow advancement after 3rd level...a lot. In 3rd edition, there was a project to cap leveling at 6th, then apply minor benefits (e.g. extra hit points or an ability) each "level" thereafter. It kept things within reason for that edition. Doing that for 5E, beyond my pay grade.I am somewhat concerned, however, that even if I found the perfect module, or set of modules, that the tone would completely change once the party gets up in levels (probably around level 5), when the feel of it might change. So, I'd be interested in ideas around how to deal with that. I know there are a lot of variant rules, or rules adaptations that might be applicable there. So, also interested in hearing thoughts on that aspect as well.