One of the fun things about worldbuilding for a fantasy or paranormal novel is that you can take bits and pieces of religions and mythologies to build your own world. Popular writers like Kevin Hearne have had confrontations between their main character and the gods of Greece, the Norse, and the Celts as well as demons, angels, werewolves, and vampires.
This mix and match can be as much fun as an a la carte dessert tray.
However, and this is a big one, you must include the light/good and the dark/evil elements of these choices so that the playing field isn’t ridiculously one-sided.
One of the most common mistakes I see is the use of only the dark/evil part of a pantheon or religion.
A recent young adult novel I read had Judeo-Christian demons invading this world with only a small number of magical characters to fight them. The two most powerful humans were a couple of ten-year-old boys.
I kept expecting some force from the light to make its appearance to help give these kids and the human race a chance, but none appeared. Any major victory without help is ridiculous and unbelievable.
Consider the show SUPERNATURAL. The universe in this series has both angels and demons in play. The angels, for the most part, are “big dicks,” but a few offer some assistance in the constant struggle against demons and other monsters. Sam and Dean have more than themselves in this struggle.
SLEEPY HOLLOW has a demon and Death, one of the Four Horsemen of the Apocalypse, as well as the monster of the week, but there is a light/good side not only in the good guys but in subtle hints that God isn’t missing in this fight for human survival.
As writers we must stack the odds against our heroes so that their victories are sweet and hard fought, but we can’t make the mistake of making that victory ridiculous by offering no help from the the light side.