Based on the movie title, I expected demons being slaughtered in underground caves. Instead 95% of the film takes place in a suburban home where five people taken refuge after some sort of unknown disaster occurred. I suppose technically the title is correct since much of that time is spent in the basement, but that's not what you'd expect.
Dave, a horror film maker, decides to use his video camera and the film he has left to capture the craziness that's broken out in town. When he stupidly ventures out to see if a woman in the yard is okay, she's not human. On the way back to the house, he runs into another woman who needs a place to stay and invites her inside. Soon a man is banging on the door needing help, and later they wave over two people trying to outrun the creatures outside.
The characters are stereotypes: the guy who films everything; a stripper with a heart of gold; her abusive ex-boyfriend; and a Christian couple who doesn't like the others because they're heathens. Later in the movie, the music swells as several characters provide sad back stories. It seems really out of place, and there's nothing original in their stories.
The only way we know the monsters are demons is because one character refers to them as such. Otherwise they appear to be quick moving zombies with sharp teeth. Following typical zombie lore, anyone bitten by a demon will eventually turn into a demon. Plus the cover is totally misleading since there is nothing like that in the film. It's a low budget production and the demon makeup is pretty minimalist, and will probably be mistaken for zombies other than the teeth.