Out With the Old
During the Dawn War, a powerful god sought a weapon that would elevate him in might and esteem above all gods and primordials as well. Through some lore unknown to any but the gods, he acquired a shard of substance wholly alien to this universe, a potent artifact known as the Seed of Evil. The Seed whispered to the god, enticing him to “plant” it in the Astral Sea, where it would warp the Lattice of Heaven and spread throughout the souls of all races. But this god was vain and treacherous, even toward his own creation. In a gambit to be the Seed’s master rather than its pawn (despite already being irredeemably corrupted by it) the rebel god took the Seed to the “bottom” of the Elemental Chaos, its farthest reaches, and buried it there.
The effect of the Seed on the already-mutable Chaos was worldshattering. The Seed created a vortex of annihilation that sucked in anything that drew near, including several Primordials. For the longest time, the Abyss was a void into which many things entered by nothing returned. But eventually the lost Primordials returned, but with new forms and new names, and corrupted from their original purpose. These “fallen” Primordials became the first Demon Princes, and include such names as Pazuzu, Orcus, Dagon, Demogorgon, Gargash, The Queen of Chaos, Obox-ob, Pale Night, and Baphomet.
The Abyss is a collection of lesser elemental realms — minor dominions, elemental planes and demiplanes, all warped together by the instability of the Elemental Chaos and the corruption of the Seed of Evil. Common lore holds that the layers of the Abyss are theoretically infinite, but others assert that there are only 666 layers at any time, and that new layers are drawn into the Abyss only when one of the lowest Layers is finally consumed by the Seed.
The physical dimensions of the Abyss and its layers are hard to conceive. The “physical” locations of the Abyss within the Elemental Chaos are represented by a vast, possibly infinite series of planar islands swirling in the vortex, all being drawn into the distant singularity that is the Seed of Evil, surrounded by a black miasma that at once resembles a curtain of black smoke, a heat shimmer, and a transparent illusion of a black field shot through with sullenly glowing veins.
Entering the Abyss from the Elemental Chaos is a tricky endeavor; you must navigate to a position above the vortex and descend into its “eye.” As a you pass into the miasma (the planar border) you emerge in the skies above Pazunia. Then it’s just a matter of not falling to your death. No matter how anyone attempts to approach it externally, one can never draw any nearer to the Seed; it is accessible, if at all, only from within the Abyss. Likewise, you cannot select what layer you enter by attempting to enter the vortex from the side at the appropriate level; many who enter the maelstrom from the side simply never return (that is true of entry into the Abyss by any means, of course, but far more likely in this case) or find themselves cast onto a random layer. Approaching the maelstrom obliquely at a “lower” point does result in reaching a “lower” layer, but a hapless traveler could end up more than ten layers away from his intended destination.