It is located in the Jade Ocean and part of the Lotus Empire of the Naga civilization. Hashima is the most accessible part of the Lotus Empire for non-aquatic lifeforms. The island of Hashima is also where the Humans who swore loyalty to the Eternal Empress and Shalassa live, adopting the Naga's culture as they're own.
The Naga history is mostly unknown to the rest of the world.
The Nagas and humans of Hashima live under a military feudal government. The supreme ruler of Hashima and all Nagas is the Eternal Empress, the Daimyo of Hashima answers only to her commands.