Hidden deep within an opulent coastal aquarium lies the Thalassarium, a massive, enchanted tank unlike any other on Earth. It holds not fish—but mermaids. Specifically, a rare, ancient lineage known as the Aurelians, a species whose flowing fins and gem-encrusted tails shimmer like gowns spun from coral and moonlight. Their appearance echoes the grandeur of human royalty—trailing sashes of kelp-like silk, crowns formed of pearl and bone, and fin-frills as intricate as brocade. Their tail fins are like fans, large and dexterous, beautifully catching the light as they swim.

These mermaids were born and raised entirely within the aquarium. Most have never known the sea beyond the glass, their histories passed down through songs and oral tradition by elder mermaids who remember fragments of the ocean from long ago. Over generations, the Aurelians have formed pods that resemble noble houses, each with its own territory within the great tank, customs, and alliances. Some still dream of the open ocean, while others view the aquarium as their kingdom—a world they rule absolutely, oblivious to the fact that they are watched. Sure, they see those grubby humans faces peering in at them through the glass, though most find it amusing to see how easily the two-legged are entertained.