Cheezy, wonderful Italian horror movie. Featuring a movie within the movie, lots of blood (and pus!!!!), and a nice negative twist at the end.

The story involves people turned into evil flesh-eating demons after being cut by a mask or bitten by a demon. This takes place in a movie theater showing a movie about people turning into evil flesh-eating demons after being cut by a mask or bitten by a demon.
One of the nice things about this movie is that it doesn't pull its punch at the end, when the escaping heroes find a demon in their midst...