The "locust god of darkness?" Ooooooh, sounds terrifying. NOT. If you'd like to see a bunch of Grudge-looking people terrorizing some poor saps running around an abandoned prison, then The 8th Plague might be for you. The tagline, "it doesn't matter if you believe in hell, because hell believes in you" just makes me think of Mother Abigail pointing at Nick in The Stand. I've obviously seen that movie too many times. The 8th Plague is about a women named Launa who is expecting to meet her sister Nikki at their parents' gravesite on the anniversary of their death. When Nikki doesn't show, she goes into a panic and tears up their apartment looking for clues to her whereabouts. She remembers that she is camping, and that the last she heard from her, she was talking about visiting some abandoned prison. Let me tell you, if my little sister called me about visiting some old jail while camping, I would literally choke her right through the phone, but that's just me.
So she and her roommates take off at first light into the woods to find the campsite. They find it alright, but no one is there. They go to the local sheriff's office, which is straight out of The Texas Chainsaw Massacre where some old dude cutting his nails informs them that you can't report a person missing until they're gone for five days. Especially if they are college students and could just be off on a drunken binge somewhere. Nice job protecting the community, good sir.
Some poor deputy gets stuck hauling the kids up to the abandoned prison to see what's up. On the way there, he picks up a guy who used to work at the prison, who he thinks can help out. When they enter the prison, madness ensues, and the next 30 minutes is a mishmash of cannibalism, eye gouging, and speaking in strange languages in a droning Evil Dead-type voice. The former prison employee confesses that he was aware that the prisoners were worshiping some "locust god of darkness." He thought it was just a way to pass the time, but oooooops, they were serious.
So it seems the cause of all the craziness is that if you look at some weird symbol drawn on the wall and you go all bananas. It's never made entirely clear what this is all about or why, which is pretty much why this movie fell pretty flat. I'd pass this one up if I were you.