Clashcradyne is the name for my demon-occupied headworld, and is what 95% of my illustrated art is about. The name doesn’t mean anything and doesn’t actually appear in-universe.
In this setting, there are three universes: primal (humans, plus any extraterrestrials sharing it with us), ethereal (fae), infernal (demons). Travel is possible between the three universes. All humans can withstand existence in the ethereal and infernal worlds without immediate harm, but not all demons and fae can exist outside their own universe.
All three universes contain magic. The reason our universe has apparently no magic is that it is vast. The magic is spread so thinly it’s unusable. The ethereal and infernal universes are tiny enough that magic is dense enough to exploit as an energy source.
Humans do possess a nonzero amount of magic, and it is the magical anus smell. If you scrounge your fingers around in your buttcrack through your clothes, some of the smell will somehow transfer to your fingers, even through multiple layers of clothing. This is physically impossible and the process is entirely magical.
The enemies of demons are not the angels (angels are just demons with good PR). The enemies of the demons are the fae. Humans are overall neutral in this conflict, though there are of course humans who make deals with one side or another. Demons allow themselves to be summoned by humans on condition that the humans help them if the demon world ever goes to war again.
To the casual observer, there is perhaps not much difference between a demon and a fairy. Both are eldritch monstrosities and rather heartless by our standards. You can form contracts with both (not at the same time; that won’t end well), both will probably find it funny to trick you, and both will probably exploit any loopholes possible in your contract. The main difference is that demons can be summoned by humans thanks to an old pact between the two worlds. Most humans have forgotten but demons have not.