Avernum was a prison, an enormous warren of tunnels and caverns far below the surface world.
The surface is ruled by the Empire, the all-powerful masters of the known world. Misfits, petty criminals, and those with the wrong opinions were sent into Avernum for the rest of their lives.
Then the Avernites assassinated the leader of the Empire and spent years in a brutal war with the surface tyrants. The Avernite rebels won. Now, at last, they are beginning their campaign to leave their caves and return to the surface.
You are the scouts. You will be the first envoys to the surface. Instead of the sunlit paradise you dreamed of, you will find a world ravaged by mysterious plagues and hordes of bizarre monsters. Soon, the Empire will ask you to help solve the problem. Your enemies will become your allies as you race to find the source of the destruction.
Fail, and you will never return to the surface. There will be nothing left to return to.