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The Abyss' was the birth place of the demons, a chaotic evil universe unto itself with uncountable layers of infinite variety connected haphazardly. The Abyss's place in the cosmology of the Forgotten Realms has shifted over time but the nature of the plane remained fairly constant: a violent, malevolent place where the strong survived on the backs of the weak, the weak conspired to overthrow the strong, alliances only lasted while convenient, and the landscape itself tortured the mind and body of all who dared to pass.

Cosmology
In the Great Wheel cosmology model used in first and second edition D&D, the Abyss was an Outer plane on the Great Wheel connected to the Astral plane, Pandemonium, Tarterus, and Concordant Opposition. The first layer, named Pazunia after the demon lord Pazuzu, was described as a barren, dusty place baking under a red sun, broken only by great iron fortresses, huge holes in the ground, and the river Styx. Some branches of the Styx flowed down the holes (which were conduits to the various levels) and some of these holes supplied water to the Styx in reverse waterfalls.

When the Great Wheel model was replaced by the World Tree cosmology model, the river Styx was renamed the River of Blood and it flowed through the Blood Rift&mdash;an unusual plane that connected the Abyss with the Nine Hells at the top of the World Tree, bringing the demons even closer to their arch-enemies, the devils. The age-old feud between these fiends became known as the Blood War. The Astral plane connected each of these Fiendish planes to the Prime Material Plane, but not directly to each other.

After the Spellplague, Asmodeus consumed the essence of the fallen Azuth and ended the Blood War by casting the Abyss to the furthest depths of the Elemental Chaos. The World Axis cosmology described the ruined Abyss as a domain with ever-changing features and realms, where the demons futilely waged wars over meaningless empires. Once again the river Styx flowed through the Nine Hells and the Abyss, but then emptied its pollution into the Astral Sea.