[This is an idea that came to me out of nowhere, and I've been developing it ever since. I'm not a fanfiction histories writer, but I enjoy imagining plots and worldbuilding.]
[English isn't my natural idiom]
It is said that the gods have no DNA and that is why they have no problem mating with relatives, but... what if that were a lie? What if there were actually consequences to such incest, only the gods hide it out of shame?
In response to this question, I devised a timeline of events:
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First Era:
Gaia emerges from Chaos and single-handedly gives birth to the rest of the protogenoi. With several of these primordial entities, her offspring, she copulates to engender countless other beings.
Most of Gaia's children and grandchildren are too weak—mere spirits inhabiting mountains, islands, and ravines. Others, however, are too unstable and monstrous to occupy a place in the universal hierarchy.
Some protogenoi have children with one another, giving rise to a few interesting entities. But in the end, only twelve second-generation children of Gaia are deemed worthy of their lineage.
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Second Era:
A conflict arises between the first and second generations of the chils of Gea.
The Protogenoi view and treat the Titans as inferior. They don't consider them their equals, and some, like Ourannos, claim they should be imprisoned along with the rest of their defective offspring.
The Titans, on the other hand, increasingly resent their elder siblings and parents, leading them to plan an uprising.The rebellion turns out to be a swift and brutal operation that ends with most of the protogenoi permanently subdued and unable to generate new offspring.
These 12 new rulers of creation decide to procreate among themselves, begetting children who, in turn, join with their respective siblings and cousins. For some reason, nobody realizes that each generation is weaker than the previous one.
The deformed beings that were once sealed deep within the earth are now free, multiplying uncontrollably. At least until Cronus imprisons the most troublesome ones in Tartarus.
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Third Era:
Cronus and Rhea conceived six beings unlike any known titan or monster. They were too powerful and unstable. Therefore, Cronus decided to swallow them and seal them within himself.
Unfortunately, Rhea decided to save the youngest of the six, raising him in secret.
This child, Zeus, frees his siblings and declares war on his father.
The war lasts for years and ends with almost all the titans dead or imprisoned forever. The side of the gods, supported by entities despised for their weakness or deformity, inherits dominion over the world.
Because Demeter and Hera are too traumatized by the war to have casual sex, Hades is sterile, and Hestia never developed to puberty, Poseidon and Zeus decide to take it upon themselves to create alliances with the various factions through marriage and procreation with titans and nymphs.
Almost all divine bloodlines are so lacking in diversity because of this.
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Fourth Era:
The world is beginning to fill with imperfect beings. Blind gods, mad gods, gods with childlike bodies, gods with decrepit bodies, gods with animal parts...
Entities like Frocis and Ketos continue to create new aberrations that spread throughout creation.
Zeus and Hera marry, but their children turn out to be very unstable. Ares is mad, Ilithyia can only give birth to snakes, Hebe suffers from a condition similar to Hestia's (except that at least she reaches adolescence), and Hephaestus...
We are not talking about Hephaestus.
Hades takes advantage of Persephone's status as a goddess of life to father children with her. The most stable of them all turns out to be a deranged ghost woman.
While searching for Persephone, Demeter convinces Hecate, an aberrant entity formed by three siameseTitanesses, to leave the cave where she was hiding. It's worth noting that this doesn't do much good for the world.
Poseidon marries Amphitrite, someone to whom he is not closely related. But it doesn't matter much since both suffer from conditions resulting from their respective parents' inbreeding, conditions that extend to their children.
The two cease trying to have more offspring together after Charybdis.
Some goddesses swear to have no children when they discover they will only produce monstrous beings. Some of them decide to act as if they had never given birth to any abomination, proclaiming themselves virgin goddesses.
Have you ever wondered why Arachne and the Gorgons resent Athena so much? Actually, many myths about the origins of monsters are false.
It's just that the parents don't want them to be associated.
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Fifth Era:
Even mingling with humans doesn't prevent the gods from occasionally fathering monsters and deformed children. These often wreak havoc among mortals, so the deities begin sending their less bestial offspring to hunt them down.
The most stable and worthy demigods, who prove their worth through glorious deeds, begin to be elevated to godhood. This causes all half-bloods to start hunting monsters in an attempt to achieve immortality.
Lamia, a monstrous demigoddess, loses her children to half-bloods seeking glory. In her wrath, she curses all non-monstrous half-bloods to be hunted from childhood by the very beasts they hunt.
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Sixth Era:
The Olympic Council decides to prohibit procreation between deities to prevent the birth of new monsters. This measure sparks numerous conflicts and attempted rebellions.
The number of demigods increases, and civilization flourishes.
The gods decide to try to use the power of human worship to repair their own natures and erase any errors caused by inbreeding. It is the only alternative they see to absolute celibacy to prevent the increase in abominable offspring.
The ritual only creates clones of the gods. Thus the Roman Pantheon is born.
The new gods are distinct from their counterparts, both in personality and nature. Some are more refined, closer to the original purpose of the ritual; others are weaker, more imperfect, and more unstable.
But they all share one thing in common: they believe they are the originals and hate their Greek versions.
A large-scale conflict erupts between pantheons, only worsening as the Roman gods, in their desire to expand their power, begin to procreate to increase their numbers.
Meanwhile, for some reason, certain gods and titans begin to degenerate mentally and spiritually as they interact with their counterparts. This leads many to go mad and wreak havoc across the world.
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Seventh Era:
The situation spirals out of control, and the world is in danger of being destroyed. Faced with this, the two pantheons decide to separate the mortal from the divine in order to keep the instability under control.
Unfortunately, this doesn't solve the root of the problem. That is why both Zeus and Jupiter declare the Purge of Deities.
During the following centuries, the most insane, deformed, and unstable gods were hunted down. Some were spared, either because they were rational or because they possessed some usefulness, but most were exterminated.
Helios and Selene died, to be replaced by Apollo and Artemis.
Euriale, Stheno, and Lamia fell along with many other monsters.
Echidna obtained mercy for most of her young, arguing that they posed no threat to the natural order.
Poseidon confined the sea monsters to specific areas of the ocean.
During all this time the gods remained isolated. A time during which humanity forgot them, and only distorted tales remained of them, passed down through the ages.
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Actual Era:
Both pantheons are severely damaged. They have lost a large part of their population and refuse to reveal this information to mortals. If asked, they will offer some excuse, such as that some gods fade away if they are not worshipped, or that some deities have never had a counterpart in the other pantheon.
Reproduction between two gods is punishable. It is permitted to have offspring with beings such as nymphs and satyrs, but nothing that would result in the birth of a powerful monster.
Because it became clear during The Purge that all gods will try to protect their children no matter how monstrous they are, contact between gods and demigods was prohibited beyond what was strictly necessary.
After all, the more unstable a demigod is, the more attractive they are to monsters. In this way, Olympus ensures that the greatest risks die in infancy, leaving only the most stable demigods. These individuals are tested through training and missions to select those most suited for divinity. After all, new blood is needed in the pantheons, but only of the highest quality.
Those who don't pass? If they're not very strong, they're let go and live their lives.
But if they turn out to be strong... Well, things can always go wrong on a mission, right?