Sounds interesting so far. It has elements of the basic plotline of the computer game "Fallout" ( http://en.wikipedia.org/wiki/Fallout...mputer_game%29 ) although you're going to be hard pressed not to have any overlap with other post-apocalyptic games so I wouldn't worry about it.
Let me fire a barrage of questions at it that may help flesh out some details or back-story or provide opportunities for adventure ideas:
- So the Demon Lord recovered the six of the seven orbs the Saviors of the Dawn hid?
- What is the nature of the Cataclysm? Bombardment from space? Magical interaction with the sun?
- Was the Demon Lord not capable of simply outright destroying the planet?
- Why did the Demon Lord want to bring about the Cataclysm in the first place? (Perhaps he wanted a post-Cataclysmic planet rather than a destroyed one?)
- Is the blue dome a dome or a sphere? Can something get into the city by burrowing from below?
- Does Gerald use crystals to power the sphere, or magic from the towns' magic users, or both? Is he running out?
- What was the basis of Ramastule's prophecy? Obviously he knew what he was talking about, but how? Is there some connection between him and the Demon Lord?
- Why are the living spells admixtured with [Cold]?
- What happened to the original Demon Lords that initially created the Orbs?