The Mershark Witch Princess

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FLUOOOOSH!

As Gaby fought the Aberration with her ghostly ship and her slime companion, the rest of the Realm of Atlantis was in utter chaos.

The seas were constantly waving uncontrollably, and because the Outer God's tentacles had shook the stone base of the sea, it began to change the entire course of the oceans.

Huge tsunamis started to consume every small island in this Realm, where there were no continents, but a series of islands and archipelagos scattered across many areas.

Most of the islands were simply filled with Monsters or Divine Beasts, and the largest populations of mortals were mostly inside of Poseidon's Divine Realm.

That is, mostly. Although Poseidon governed the seas and this entire Realm, there were many tribes of people that didn't obey his rules, and because he was mostly passive about it, he let many tribes develop on their own.