Plain square room with the floor textured as the combine tile floor. The catch is that random tiles will instantly rocket up into the ceiling if you step in the middle of them, crushing you instantly.
Also, the tile in front of the exit rockets up, but it has a doorway when it's extended, meaning you have to sacrifice (or murder) someone to get out of the room.
This map will be even more fun if the gore mods work on player ragdolls.
Also you need to make it so that players that die are out until mapchange.