This level can be accessed through the Spawned Fear level. To get there, after going through the Yellow Door and walking up the steps, you will hear a door open. Quickly run back through the Yellow Door and turn left and then turn left again and enter the room there (the one where you faced 3 Hell Knights). You will see a small area with a teleporter exit in it. Run for it before it disappears and that is it.

You will now be taken straight to Level 31 In The Void. If you fail to make it to the teleporter, just repeat the steps above, there is no need to restart the level.