Based on a chapter in the upcoming novel Legacy of Prometheus. Old version, of course.

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It has been a long time ever since the end of Second Prometheus War in Abraxis. The Empire's defeat has been a final blow to their manpower and pride, and it has been disbanded. Cassandra's fate is now unknown, Effyis and Anotchi having perished with the Antimatter Cannon. Most of the coalition's forces have returned to their lands, with only some Empire inhabitants voluntarily remain in that blasted land.
There's no reason to stay with the mainland and indulge in politics anymore. Ingefara and Rosenberg deserved better than that. With some knowledge on the sea, they decided to have an expedition to the depths of Persephone Sea, the one that stands between Abraxis and Amelia. No one knows why it's chosen though.
With the potions of water breathing ready, a deep dive is performed and in a matter of time, the scent of cold water has claimed them all. Corals, schools of fish and bubble columns dominate the sea, while the shipwrecks are scattered randomly. With some treasure-hunting skills, they found some old, unknown conduits, along with two of these tridents. They may appear to be of no use at the moment, but soon.
So... Why do we have to come into this place? There's nothing amazing to research besides these artifacts. Explain me why!

Well... For several reasons. According to what our historians said, this "underwater ruins" was once the heart of an ancient civilization, long before we even existed. A mystery, so everyone wants this one discovered. Fortunately we're not alone.

Then what's even the point in bringing this amulet with yourself? You have someone to find, don't you?

Just as what they feared, that one has arrived and closed in to them. At first, it's mistakenly assumed to be one of the Drowned, intending to bury them in this sea forever. They're about to prepare their weapons and fight against this menace when the latter has been proven to be wrong, as it turned out to be a human, the one with an admiral's suit. A water mage, thought Ingefara.

What you're doing here has nothing to do with our ancestors? The first greatest genocide ever, destroying all the inhabitants inside. I was the sole survivor left. This is what still remains in the face of Persephone era.

From that moment, the truth has been revealed. Their doubts are now nonexistant. In the front view of their eyes, what that water mage said was right all along. That underwater ruins are ressurected, the Ocean Palace remerged from the ashes. Finally, the Ancient Civilization of Persephone for the entire life.
BONUS
"I... I don't know. I just don't know why. Something is instructing me to this cold sea. Something that's lurking behind these corals." Ingefara replied with a mysterious tone.