A bunch of 18 year olds go to a deserted cabin for a weekend of fun. They find an old audio recorder in the basement and start playing it while doing drugs. It contains an incantation to call demonic spirits towards the cabin. One of them then goes out and is viciously attacked by the forest. When she gets back, she falls victim to demonic possession. Throughout the night, they are becoming demons one by one and the only way to stop them is by bodily dismemberment. Eventually one man remains while all others are possessed. He cuts his own hand off and replaces it a chainsaw when it gets possessed. Eventually, he kills all the demons. However, in the end, it turned out that he had done so much drugs that he imagined all the demons and just viciously killed all his friends and the others....

Unfortunately, I just recited the plot of Evil Dead with the original twist that Scott Spiegel thought of for the second one. Still, it's a generic enough plot that the new twist at the end can actually make a different horror movie if done well. You could also interchange the demons with ghosts, zombies, or even mutants for more originality.

I guess my main point is that it's not necessarily how original the idea is, but how well you can make it work. I mean even classics like The Shining weren't that original; the cast and director were just really good.

EDIT: Still, of the ideas on this thread, I do like Ziggy's. We need more horror movies in the jungle...