User blog comment:Caudyr/Abilities List From the Author + Translation of Abilities as I go Through the Story.../@comment-24863069-20151112143601/@comment-124.83.16.225-20151112145524

Well, the kanji for both can be translated into 'demon', so it's a bit trying to find ways to differentiate them. Demonkin isn't even entirely correct since one of the Fiend monsters is called a "Demon Lord".

It's a cultural thing, I'd say. Oni, that is. Oni has been generally translated as demon here, but the word itself is associated with the "Fiend" (which is just the chosen tranlation for the kanji on this page) race. So, you know, it's kinda complicated.

Going from the races we've seen in the story, Demonkin seem to be the general fantasy race monsters (kobolds, monkey demons, orcs, trolls, ogres, goblins) while Fiends have got the "devil" image (the Demon Lord monster I mentioned pretty much looks like Baphomet, but without the breasts). You could also read up on 'yaksha', which seem to match up with the general feeling of "Demonkin" in this story.