he Abyss is not truly part of the mortal world at all, but is the extraplanar home of the demon races, spawn of uttermost evil. In ages past, powerful mages created the Portals, mystical gates to this vile place, the better to summon their denizens to serve them. The demons seized their chance to pour into the mortal lands, and caused much devastation before they were at last repelled. The mages who created the Portals are long since dead, but still the Portals remain—although now battle-hardened human warriors guard them against incursion from the far side.

The Abyss proper is reached most directly by the eastern Portal, situated in the Blackgate Mountains some way to the north of New Thalos. It can be reached by a less direct route via the Mini Abyss, a pocket plane whose Portal is found in a cavern near the source of the Dark River.

The Abyss is inhabited by many terrible demons, among them the fiery balors, the many-armed mariliths, and the shadowy Black Slayers. However, demons are by no means the only inhabitants of this foul place. The Abyss is home also to other spirits of wrath and torment such as efreeti, to huge monsters such as hydrae, manticores, and krakens, to various undead—greatest among them a powerful demi-lich, an undead archmage of surpassing power—to an evil knight of great prowess by the name of Kalas, and to evil storm giants.

Explorers can travel from the Abyss to the soul-chilling Negative Plane, and through this ghost-infested place to the lesser evils of the Mini Abyss and ultimately back to mortal lands proper.