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Entities known as "Tsugai"

Supernatural beings modeled after myths, yokai, folklore, and UMAs. They exist in pairs and are summoned through contracts with humans. As a basic rule, Tsugai protect and obey humans, but some are reincarnations of the dead or possess god-like power, engaging in battles with various abilities.

Twins of Fate and "Break"/"Bind"

The core of the story involves male and female twins born at sunrise on a day when day and night are equal. By dying once, these twins gain the power of the Tsugai of the Underworld: "Break," which can forcibly undo anything in the world, and "Bind," which can forcibly close anything. The protagonist, Yuru, possesses "Bind," while his sister, Asa, possesses "Break."

The boundary between life and death

Concepts like "Yomotsu Hirasaka" (the slope to the underworld) and "Resurrection" form the foundation of this world's view on life and death. It explores the cycle of life with Tsugai and the succession of life predicated on death.