Underworld Dimension

The broken portal in the deep dark should lead to the underworld.

Mobs: All undead, endermen (they can get anywhere), wardens (the portal is in the deep dark, after all). Zombie horses and giants could also be introduced.

Lighting: Dull grey/sea green.

Terrain: Mostly flat with glowing cracks in the ground. (Based off Dan Bull's skeleton rap on Youtube.)