Appeared in: Season 5, Episode 19 - "Folie à Deux" A lot of good science fiction involves examining how we see the world around us, literally - what is real, and what isn't. Questions of reality come up in The Matrix (basically a riff on Plato's allegory of the cave), Neuromancer, numerous Star Trek episodes, the list goes on - but those are less scary and more science-y. "Folie à Deux" (which is a term meaning "a shared psychosis") from the fifth season of The X-Files brings the creep factor right along with the science. Greg Pincus is an apparently decent call centre boss - but one of his employees happens to see him as a monster, literally, who feeds off his workers, turning them into zombies that can't be seen save by a select few. After taking Pincus and other workers at the office hostage, the employee winds up dead - but not before Mulder and Scully are called in on the case, and not before Mulder becomes able to see the monster Pincus really is, becoming part of the Folie à Deux. What made this insectoid-like monster so creepy? The sounds the creature made were unnerving, and the fact that only Mulder could see it (especially when hospitalized and strapped down to a bed) - it gave you a sense that, for a change, he was actually in danger.