Demons and the forces of darkness are horror movie staples for a reason: they are horrifically terrifying. Whether you believe in demons or not, there are enough supposedly true stories of demonic encounters out there to make an endless string of movies. From adults to kids, from believers to non-believers, from those who went out looking for trouble to those who just made the mistake of moving into the wrong house, people have had truly chilling encounters with vengeful demons they've offended.

While it is generally ill-advised to pursue any demons at all, these demons you don't want to mess with are truly the stuff of nightmares. Some take terrifying forms, some inflict horrible pain, and others lend those they possess strange powers. They have even committed and inspired slayings. While some demons (such as the Bell Witch) are parts of tales that have been passed down for hundreds of years, others have occurred only recently. In 2008, a board-certified psychiatrist even documented a real-life case of what he considered clear-cut demonic possession.

What's out there is dark, it's terrifying, and here are the reasons why you don't want to piss any of it off.