Therasis is a planet that was once covered by a layer of rock and metal, making it bigger and richer for population to live. Then it became  some kind of refuge for all living people, since the Schistosomas were unable to live under the light of the sun. This... shell. was called "Asphid" 
Time passed and de shell became wasted and eroded. Plants were unable to grow anymore and the rocks were slowly turning to fine dust which stayed suspended for periods of time long enough to make the air on the surface haaaaard to breathe. People were trapped out by their own invention, while under this shell layed a dangerous utopia of wealth. 
The sun was built in the entrance to avoid the Schistosomas to get out.

First try of concept art though xP