You actually can 'summon' demons—I'm going to be called schizophrenic for this, but they do in fact exist and I've been trying to learn more about them for some time and have even made a few deals with them. However, I do have to immediately make clear that these aren't literal 'demons' as we understand them—a lot of them don't actually have origins in any form of 'hell.' They're beings that exist amongst us here on this planet, and even have their own societies. They also aren't technically called 'demons', I don't actually know what they call themselves—but I call them devils.
It's probably best I don't go too much further into detail (unless you want me to) lest I be called extremely retarded.