While everyones coming up with new ideas, i think we should have a new underground zone. As I'm sure lighting will be a major problem, you should also create a new item called an unlit torch. The only problem with that is that you would have to create another item that would light the torch. The whole reason for the torch, however, is as the zone is underground, there will be no light. There will also be places to put the torches because you won't be able to hold the torch while you are fighting. With this new zone, there should also be new creatures that corrispond to the zone: impervious to darkness, very gruesome physical features, the ability to camoflouge to the walls and ceilings, and a new status effect called fear which will lower attack power, defense, and speed. Having a torch lit and placed into one of the holders in close proximity to you will prevent this.