Magic Forums

I had a post about demons recently, but I figured I might get more attention if I started a new thread on the topic.
My question is how can demons affect reality? There are stories of demons bringing loved ones back to life, undoing a mistake you made in the past or having powers and knowledge beyond any other humans on the planet, in exchange for there soul. I should also say that I don't want anything to do demons, but I really want to know what they can do (I don't really want to meet any demons in my life, but hek it doesn't to be perpared).
THere are certain things that are said to be immposible with magic, shapeshifting, physical travel and superpowers. But if humans can't do them, can demons? Can demons give humans these powers that normal magic can't? I know some of this was answered in the previous thread I did, but I would like to know more.