The fortress city of Rivera, situated in the central plains where the Mother River winds, is a strategic hub holding the continent's lifeblood. Since the 'War of the Red Sunset,' Rivera has built massive walls and accumulated wealth by forging sophisticated weapons using mana crystals salvaged from the riverbed. However, life outside the walls remains perilous. By fending off mutants crawling from the upper reaches of the river, Rivera continues a precarious tightrope walk between tension and peace.

The terrain surrounding Rivera is harsh. To the northeast, the 'Black Claw Tribe' lurks on the 'Twilight Cliffs,' driven by hunger to constant raiding, with the 'Silent Forest' to the east serving as their hideout. The 'Salt Desert' at the southern tip is a dangerous treasure trove where remnants and artifacts of ancient civilizations sleep, while the 'Misty Hills' to the west are a battlefield for complex political intrigues between city-states. Ultimately, Rivera is both a shield against barbarian invasions and the heart of the continent where southern artifacts and western goods converge along the river. When the golden sunset reflects off the water, transforming the city into a gilded fortress, the people take a bated breath of momentary peace, briefly forgetting the clouds of war looming for tomorrow.

The six registered characters are all 25 years old. They grew up in the same district since childhood.