Ghendar said:But why would surface dwellers want to invade? The Underdark environment is not very hospitable.
For resources. Maybe metals, minerals or elements that are rare on the surface, but available in relative abundance right under their feet. They don't have to invade to conquer the entire underdark, just a portion that they see value in.
Once they get what they want and begin mining it or whatever, perhaps they unleash something...problematic, that leads to the next stage of the campaign. Miners going missing, confined terrain, darkness, sounds like the makings of a scary movie.