This throaty, frightening voice is what you want to use to portray a demonic character. The quintessential demonic voice may be Regan's from The Exoricst, but it can also take on a variety of specific tones to scare the pants of its listeners.
A demonic voice reaches far into ourselves to find what scares us the most. The voice is perfect for audiobooks or films to showcase the evil tendencies for the character. It can be amazing what a voice can invoke, and the demonic voice can certainly invoke fear, horror, and that little bit of beloved chill that listeners want.
There's a certain crowd that loves to be scared. Whether they are watchign a horror movie, listineting to a scary podcast or audiobook, or playing a video game, they seek the thrill of fear. This voice may be too much for the young child crowd, but older groups will love it. And for those that love the demonic voice and character, they really love it, so make it good!
When your evil character speaks, his or her voice makes terror and fear and evil audible. Give them a demonic voice to showcase that evilness embodied. This voice may be throaty and slow spoken, maybe deep, hoarse, and gruff. Think of a loud whisper laced with hate. A demonic voice is often genderless and otherwordly; evil is its primary characteristic. Though the demonic voice isn't hard to understand, there's something about it that makes the listener know it's not natural.
The demonic voice is evil emodied, don't be shy with it. Go all out and give your listeners those chills of terror they're seeking.