I mean...the plot sounds good for a high-level campaign, but...that's not really "world building". We're going to need a lot more.

What was the town before it was overrun? What was special about it, that the demons started here? Why are the PCs there?
What else is nearby? Who else is nearby? What other NPCs will get involved?
Who is Manus, what are his character traits, who are his allies, who are his enemies?
What's special about this demon invasion that can destroy the world? Demons wander worlds all the time without destroying them entirely.
What about the surrounding lands? The country? The continent? Who's in charge, what deities are worshipped? What races are prevalent?

The plot is fine, but I'm going to have a hard time commenting on the campaign when all we have is one town with no known features.