The Abyss is a section of exile from Avernum. It is ironic (but sometimes necessary) that Avernites, who were exiled from the surface, would choose to further exile criminals deemed too dangerous for life in regular exile. Its two main cities are Bargha and the Spire.
The Abyss is protected from Avernum by the Spire Fortress. It was a one-way passage for criminals who had been banned by Avernum. After the Empire stopped sending people to Avernum, Avernum stopped sending people to the Abyss.
One of the most skilled (and secretive) groups of fighters was the Scimitar, whose main base was in Bargha.
The Abyss was the first (inhabited) place that was overrun with Empire troops during the Empire-Avernum War. Many of those who didn't flee were slaughtered or forced to aid the Empire as spies, soldiers, slaves, etc.
After the Empire War, many Abyssites came back to rebuild their home. They have (for the most part) made peace with Avernum, and can be seen as a sort of protectorate.