The Nemean Abyss is a region of space that is further away from the jurisdiction of the Citadel Council than even the anarchistic Terminus Systems. The Abyss is a lawless region, populated by pirates, mercenaries and bounty hunters.
According to Major Derek Izunami, the Abyss is the darkest part of the galaxy, where humans rarely venture, partly due to the sizable batarian presence in the Abyss. However Miranda Lawson says there are still a few scattered human colonies, mostly pioneers and those looking to escape the overview of the Systems Alliance.
The Abyss is also noted for being rich in element zero. This has led to wild stories that there are actually drifting starships in the Abyss made of element zero. While these are likely exaggerated, there are certainly cases of ships being captured by pirates while carrying huge eezo shipments.