Demons/Dèmoni (1985)

A mysterious man in a metallic mask is handing out free tickets to the premiere of a film entitled "Demons" at a new cinema renovated from an old gothic structure. Cheryl and her best friend decide to skip music class to see the movie. A bald black pimp and several of his whores are bothering people in the lobby. One tries on a demon mask, and it cuts her face. The cut will not go away, and eventually starts to fester, turning her into a green-drooling demon, which transforms people with her nails. Two guys sit down with Cheryl and her friend viewing the film documenting a similar case to what is happening in the restroom, associating it with the prophecies of Nostradamus. Eventually the theatre becomes a walled-in trap, and those who survive must rely on some disgusting punks who snort coke from a Coke can to come to the rescue, riding their motorcycles around in the theatre. The guy who prefers Cheryl becomes a gun-toting Ash-like hero, while the others transform into demons for the slaying one by one.

IMDb: https://www.imdb.com/title/tt0089013