I like what someone said about taboos or unwanted things being the basis for swear words. I'd also keep in mind that they have another name--curse words. Anything that your characters might say to 'curse' another person could serve as an insulting slur, and if you're creative enough with english you could avoid @Wriybies comment that contrived swears are absolutely infuriating (which is totally true and I agree on all accounts). I recently finished Master of Crows by Grace Draven, where they establish the book-long swear very early. They don't give elaboration on who "Bursin" is, but because the swear is "Bursin's-wings", it sort of follows after you read it in context a few times that its a swear, or rather, equivalent to saying "Jesus Christ". This is further expanded on when the less refined characters start saying "Bursin's Balls" to curse. It's the only real curse word in the book, and even though it was clunky at first, I grew to accept it. I also like "skies" as something to shout out, as though your character is exasperatedly screaming to the heavens. But I think, in fantasy, as long as you can justify it you can make it work. Sometimes though, it's better to stick with what your audience knows and just incorporate the idea of "hell" into your lore somehow hah. Edit: As an after thought, since your language your characters are speaking probably isn't 'english', you could always take the really quick and easy way out and say "Oh I'm using english swear words so you all get it, but in actuality they're swearing in their own language." Bullet dodged.
What do your people find disgusting, distasteful, or dirty? Those are whence swears spring, after all. They can be pretty culturally specific, after all. A culture that thinks highly of dogs, for instance, might find the phrase "son of a bitch" to be a _compliment_.
I think invented swear words can sometimes be overdone. I am thinking of Dashner's Maze Runner series with its whole dictionary of "Glader Slang" such as slim it, slinthead, or shuck. Somehow it really, really annoyed me every time a character says shuck this or shuck that. It just sounds really unnatural. Why couldn't they use four letter words that we know better? So I am of the opinion one should use contemporary swear words.
