The 7 Planes of Hehenna is the 3rd area in the game. This is where you can find lava for the first time. Here can be found such enemies as Hell Nibblers and Gimps. The bosses for this area are Vis Sisters.
There's also a secret warp zone in this area (also the hardest one in the game). On a level 3-4, there will be columns of skulls shot up from the ground. You have to stick to the first one, fly up, and touch the '666'.