I like stories which flip the Hero - Villian dynamic too.
Though I never really thought Hades evil, every story I've heard of him he always just gets the short end of the stick.
The poor guy had to give his dog to Heracles just so his nephew could do one of the twelve labors.
So we can start here for a plot...
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Sparta has interesting stories too.
The whole 300 battle with the Persians.
The Peloponnisian War is one of my favorite historical conflicts.
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Sphinx was Egyptian I though?
Well, both were close in proximity so maybe they had similar cultural ideas.
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I never read Daugher of Sparta.
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Oh! I have an idea for a plot!
Our characters serve the Greek Gods.
Either due to them being demigods or regular mortals who serve their patron Gods, they embark on a quest for them.
Basically the Peloponnian War is in full swing, Greece a war torn waste as Spartan and Athenian clash... And all the Gods blame Hades.
It's only logical, the more death, the more his Kingdom grows.
Our characters could be a Spartan and an Athenian, forced to work together to solve the conflict between both Greek super powers by journeying to the Underworld to convince Hades to stop the war.
During the journey we can encounter various mythological figures like monsters, gods, other heros, stuff like that. Along the way we begin to learn the Gods might not be who they truly are...
Once we reach the Underworld...
According to out mutual liking of the trope, the culprit for the war is not Hades.
Everything else is a mystery from there. :D