When I think of the creepiest Halloween party themes for adults, I think of all the Halloween parties that I have been to in a large field surrounded by woods. They take a long time to set up, but they can be fun. It also helps if someone you know has lots of land, and is open to hosting a party like this.
One party that I remember had a trail that was cleared through the woods with a four-wheeler. All kinds of creepy props were strewn about in the woods with jack-o-lanterns lighting the way. Things like open suitcases filled with burned and bloodied dolls that had all their hair cut off, scarecrows tied to trees, Mounted dear and bear heads from my buddies basement that he hung on trees, and lots of fake cobwebs. To top it all off, there was a roasted pig and roasted chickens on a table covered in fake blood. The beer was in coolers next to the table, and fake blood was mixed into the water and ice in the coolers.
In the field were a bunch of tombstones made out of painted plywood. The host also started the party by torching a bonfire that was covered in straw and doused with gasoline. It was a great night, and the trail through the woods was definitely creepy.