Are demons real and if not then how did they come about?

My friend and I are going to write and shoot an independent horror movie with two main characters, each with their own separate stories. It’ll just be in one house location with two rooms for rent. He will write for a male character while I do the female.

We agreed to come up with plots never been seen before in American, Korean and Japanese films. I chose to include a demon in my story. Apart from watching Hollywood demon movies, I’ve never done research on them. I thought I’ll start and maybe end my research in Fluther. Come up with a Fluther demon maybe? : )