The 523rd layer of the Abyss is a realm of ruin and destruction. It is filled with ruined buildings, twisted remnants of civilizations past. Throughout the buildings, a unique type of demons roam. These demons are unique in that rather than being elevated from Larvae, they are the twisted results of mortals and feinds alike when altered by the essence of the layer.
The first to undergo this process was the layer lord, formerly a pit feind named Karkalos. During a Blood War attack, he was badly wounded here, and left for dead by his troops. Furious, he swore his revenge upon them, and upon society to all. The layer was listening, and it granted him his wish, filling him with demonic power, twisting him from a baatezu to a tanarri. And not just any tanarri- he became the demon prince controlling the layer.
He then began visiting the other tanarri that were scraping a living on the plane, and used his new found power to make them into his minions. He then opened a portal to a Prime world, soon destroying its government and society. Pleased with his work, he took the most brutal of the bandits that sprung up afterwards with him, warping them into apocalypse feinds as well, but not before ensuring a cult dedicated to him was seeded amongst the survivors. And then he repeated his performance on the prime worlds again and again, taking what petitioners arrived and warping them into apocalypse fiends as well.
He then began raiding the Grey Waste for prisoners: tanarri, yugoloth, baatezu, night hags, mortals, whatever. He didn't care, as long as his forces grew, and none of them was a threat to his personal power.
Today, the layer is filled with roaming bands of apocalyspe demons who fight amongst themselves as often as their enemies, except for the occasional raid organized by Karkalos himself. He is happy with this state of affairs, for the anarchy keeps his armies strong, and their tendency rape their victims as often as kill them produces as many new feinds as the fighting kills.
I don't know if I'm allowed to post the Apocalypse Demon template, found in d20 Apocalypse. Can one of the PW folks reply with an answer to that?
I don't think so. It's not OGL
But this is a nice story and a layer of eternal apocalypse fits modern planes perfectly