Here's my rule about creating supernatural creatures in a story-make up your own rules. Just stick to them.
It would be very easy to research some mythological data and use these monsters, gods or demons in a usual way. Use just those same old boring perspectives. But how much fun is that? You must free your writing mind to all kinds of possibilities. Go beyond your perspective of religion, the supernatural or demons of your own upbringing. Research other cultures and histories. Study different ancient religions and philosophies. Read other novels of the types of fantasy or horror or science fiction that interest you.
You need to do this to stretch your mind.
I like making up my own dark gods and demons, good gods, monsters, spirits. After all, I am the creator of my fiction fantasy universe, so I do what I feel works best for me and my story arcs. Remember these things are also part of the flesh that makes up the culture and society of the characters. It's all important.
But when it comes to the demons and scary stuff-that is where I have the most fun. I decided to make my goblins a mortal demon race, ugly and wiry with buggy eyes. They have little intelligence, but are vicious fighters. Just the sort of thing the bad guys would hire for their armies. My trolls are stinky and stupid, but a terror of strength and power. My wicked Obsydia, whom you shall soon see as the reigning evil in the soon to be released Tree of Bones novel, is the most classical of my dark creations. She is an evil goddess who can also be seen in my short story, The Bloodstone Queen, still available to be read at Aberrant Dreams webzine. Obsydia is half mortal and half god. She is stunning in her beauty-hair of living shadow, silver-eyed, pale of skin and red lips. Very fairy tale, but deadly. She will serve, along with Koll the Sorcerer, as the chief adversaries in my Familiar's Tale series.
Poor Runa and Mellypip will have their work cut our for them.
I create a template and stick to it when I make up my evil gods and demons. It's my own personal demon code. I name them, flush out what they look like, give them strengths and weaknesses, full background and relations, all the stuff that gives stability in my fiction world. I have used the images and ideas from my study of mythology and legends, and then I adapt them to something new. It is a kernal that grown from my imagination and polished with words. There is a wealth of material out there to be scavenged-so there is no excuse. But there is responsibility-a writer must be consistent. Once that is established you can let your imagination go wild.