Even if you haven't seen Blair Witch, you probably remember the stellar marketing. In 1999, the internet was still new and people were much more likely to believe whatever they came across was a fact. The filmmakers used this to their advantage and created a website that perpetuated the idea that the Blair Witch legend had been around for centuries, along with pages detailing backgrounds of the "missing students" and interviews from their "families." The online efforts were also supported by physical missing posters the studio placed on college campuses.
Since then, viral marketing has had to adapt in order to actually make an impact, but even recent films have tried to adopt the guerrilla marketing methods. 2015's #CharlieCharlieChallenge, a Ouija-board like game where people summon a demon named Charlie, was actually a promo for The Gallows.