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The Infinite Layers of the Abyss, sometimes called the Abyss or the Infinite Layers, is a chaotic evil realm in the outerplanes and home to the demons. The Abyss was born when the evil god Tharizdun plunged a shard of evil power into the what would one day become the Elemental Chaos. Like a tree, the shard took root and created a terrible realm of darkness, despair, gloom and evil. This is the Abyss. 

The Abyss contains an estimate 666 levels, but the exact number is unknown. Unlike the Nine Hells of Baator, there is no hierarchy, no organized rule over the Abyss, just demons trying to carve power for themselves. The demons are not out for power and conquest like devils, but instead seek only death and destruction.

However, several demons have managed to gain great power and influence. The monsterous, ape-like two-headed Demogorgon, Prince of Demons, has risen to command the fear and respect of deities, demons and devils.  His ally, Dagon, Lord of the Depths, is a monsterous gargantuan sea creature resembeling a tentacled eel, and was the first demon to appear in ther Abyss, ruling its dark seas.  Orcus, Prince of the Undead, rules over a level of the Abyss filled with demons and legions of the undead and seeks to supplant the Raven Queen as the new God of Death.  The handsome and diplomatic Graz'zt, the Dark Prince, rules three levels of the Abyss. The alliance of Demogorgon and Dagon war with Orcus and Graz'zt, who war with each other and other demons for control of the Abyss.

Still, even non-demons have gained power in the Abyss.  The drow goddess, Lolth, goddess of spiders, shadows and lies rules a region of the Abyss known as the Demonweb Pit.  The demigod witch Iggwilv, one of the most powerful and evil spellcasters ever, manipulates politics of the abyss.

The great powers of the Abyss keep each other in check, waging war on each other, with some demons managing to occassionally gain small kingdoms of their own.  However the klurichir demons, born in the deepest level of the Abyss, nearest to the shard of evil, lead unclaimed demon armies against other powerful demons helping the chaos of the Abyss. 

So great and terrible is the power of the shard, that it corrupts all who enter the Abyss, transforming them to demons. Demogorgon, Orcus, and Dagon were primordials before entering this dark realm, while Graz'zt was an archdevil of Baator, and Lolth resembled an elf before the dark power of the Abyss converted them and changed them. The Abyss itself creates the demons.  If one is killed, their essences returns to the Abyss to be reborn from the darkness and the evil.

The Infinite Layers of the Abyss is a dark, terrible, nightmare-filled demonic evil realm. There is no rhyme or reason to the layers, no consistancy in geography. It ranges form burning deserts, to poisonous swamps, to mazes of razor sharp rock, to frozen wastelands, to lava-filled volcanic hellscapes, to vast oceans and lush jungles.