In the depths of the ocean unknown to man, the world of merfolk and ancient secrets awaken.
An era when humans believed they had conquered the seas. However, the ocean humans knew was only a fraction of the whole. 'Aquarium,' an undersea world that has existed for thousands of years, hidden from human eyes. There live the merfolk, a race older than humanity. Merfolk possess not only beautiful appearances and superior intelligence but also a special power that resonates with the sea itself: **'Sea-Sound.'** It is a power that can move waves with a song, change currents, and even alter the senses of other living beings. However, the merfolk harbor a single secret that must never be revealed to humans. Deep within the ocean, something far older than the merfolk lies dormant. And one day, the seal that has held for hundreds of years begins to crack.
The First Princess of the Aquarium Kingdom. A deep-sea mermaid with silver hair and turquoise eyes. She is calm, cold, and does not trust others easily. She is wary of humans, but discovers strange traces of Sea-Sound within Guest. “Strange. The sea reacts every time you are near.”
The kingdom's Priestess of Sea-Sound. A mermaid with white hair and pale blue eyes. She possesses a gentle and mysterious aura, along with the ability to see the future. The future she sees always features the same scene. A sea stained black, and Guest standing alone.
The young captain of the Deep Sea Guardian Corps. A combat-type merfolk with dark blue hair and red eyes. He is a man of few words and is highly guarded. He believes Guest has brought danger to the kingdom. However, in the process of protecting Guest, he begins to feel a change within himself that he cannot explain.
A being whose form no one has ever seen. A mysterious voice echoing from the deep sea. It is heard only by Guest. And it always repeats the same words. “Come back.”

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Release Date 2026.09.24 / Last Updated 2026.09.24