I had a world where there was only one deity per alignment box. There were six deities.
Evil worship was focused on arch-devils and demon-princes, which became relevant cults since there were no evil gods.
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There was no creator deity.
Here's what happened:
- Earth was a shiny blue planet with intelligent life. The intelligent life named itself "human", and discovered the supernatural, and thus gained powers -- specifically, Druidic magic and Psionic powers.
- Humans went exploring: their own planet, and the planes.
- On the planes, they met a large number of other intelligent creatures.
- Some of the creatures from other worlds decided to migrate to Earth, because their worlds were suffering from varying degrees of demonic invasion and/or undead infestation.
- Some of these creatures had Arcane magic. This was a big advantage, and humans disliked their lack of Arcane magic.
- The humans in Totally Not Rome decided to obtain Arcane magic. Unfortunately, this caused a demonic invasion, for reasons which are totally foreseeable if you understand the source of Arcane magic, which nobody living did.
- The humans from Totally Not Rome fought the demons for a few hundred years. Over that span of time, with their new Arcane magic & their Psionic powers, they constructed six great flying citadels. Over time, each flying citadel became like a civilization unto itself, with its own values & philosophy.
- One group of humans created an Epic spell which could create a source of power directly opposed to the powers of the lower planes: both anti-fiend, and anti-undead. Unfortunately it required a massive number of high-level participants, but the result was a permanent gestalt entity which should be untouchable by Abyssal corruption, and which could grant powers to a veritable army of spellcasters.
- The exact order of events is unclear, but each of the 6 great citadels learned of this Epic spell, and each wished for the new entity to embody their own philosophy & values. So there was a bit of an arms race and mistakes might have been made along the way...
- Each of the 6 citadels fell from the sky on the same day, and were lost from the world. By some great coincidence, the six true gods presented their revelations on that same day.
- Earth has been a bastion against demonic invasion for the last 1,800 years. Earth has tended to collect people from various doomed worlds -- Dwarves, Elves, Gnomes, etc. -- none of them were from Earth originally. All were fleeing the doom of their various home world(s).
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Every "layer" of the Abyss is a world that fell to demonic invasion. There are nigh-infinite layers. Earth is tottering on the doorstep of damnation. Roll for initiative!