Also, I once read a description of the Devil by a famous Islamic mysticist, which I can't recall or find the text now. But it was sure something different, and interesting.
From what I can recall, he described the Devil much like a beautiful angel of light, very pure, with cold, blue eyes. He was perfect in all ways except he was lacking in the Love principle. So he was very cold, and without compassion, but this was so he would be able to perform his job.
He described his task as destroying that which gets in the way of the wheels of creation. To test the weak, and destroy them. This sounds a lot like the idea of the Devil as The Accuser, or the one who accuses you before the throne of God, while Christ will speak to your defence.
I found this to be a more unique description, although I don't know if this would be the same being as is normally described as the Devil, or the root of all evil. It sounds more like someone performing a necessary function for God. I guess we all have to be held accountable in some way by someone with the heart to do it.
Wish I could find the text as it was very interesting.