Demons was produced and co-written by Dario Argento, and directed by Mario Bava's son, Lamberto Bava. Definitely one of my favorite Italian horror films. Lots of action, lots of splatter and make-up FX (by Sergio Stivaletti), and a great score by, Goblin keyboardist, Claudio Simonetti. It also has some great songs in the movie by Iron Maiden, Saxon, Billy Idol, Mötley Crüe, and Accept to name a few.
The basic action of the film is much like a zombie movie, only it's demons instead of zombies that are attacking people, and then once infected they also become demons. A man with half his face covered by a chrome mask, hands out fliers to a free horror movie playing at a nearby theater. By the way, the actor in the mask is Michele Soavi, who became a director in his own right, with such films as Stage Fright, The Church, Devil's Daughter, and Cemetery Man. Anyways, once all the patrons are inside the theater, one of them plays around with a demon mask prop that is on display in the lobby. She gets cut by a barb inside the mask, which later becomes infected. The movie inside the theater starts and it appears to be a horror movie where some people find a manuscript and a mask, like the one on display in the lobby, in some ancient ruins. One of them puts on the mask and gets cut by the barb inside it. Of course, they become infected, turn into a demon and attack the others. This scenario then plays out inside the theater, as the girl who put the mask on in the lobby turns into a demon and starts attacking theater-goers, who in-turn become demons and so forth. The rest of the film is a game of survival, as they find out all exits and entrances to the theater are sealed up. Lamberto Bava's best film, in my opinion, but he does have some others worth checking out too - Macabre (aka Frozen Terror) and A Blade in the Dark...and a somewhat fun crap-tacular Jaws rip-off called Devilfish.