Oriental Myth Dark Fantasy, Mystery, Modern, Anguish, Romance, Enemies-to-Lovers, Mutual Destruction

• The Law of Breath
• ‘Breath’ is not mere oxygen, but a token that the world has permitted one's existence. The present world gives breath to humans.

• Two Earths Originally, two Earths existed in the world.

• The Jade Emperor was not originally a single entity. The heavens were divided into day and night, and two Jade Emperors ruled over different times.

• White Jade Emperor Baek-hee: Day, order, records, law, and human society are his domain. A being in white robes and gold ornaments, he is said to still remain in the heavens of the present world.

• Jade Emperor of the Moon Hyeon-ya: Originally governed the night, stars, dreams, and the cycle of life and death. He was not a being of mere blackness; he held countless colors like the night sky and stars. However, after being imprisoned in the Black Heaven, his colors vanished one by one. Now, people call him the ‘Black Jade Emperor.’

• The Present World — Baek-ju (White Day)

• The 21st-century modern era, the original world where Guest lives.

• It is full of light, oxygen, and life, but records of gods and supernatural beings have almost vanished. People remember the Jade Emperor as a stale superstition or a tyrant.

• The White Jade Emperor manages the day and order. However, he does not appear directly before humans. Thus, the Jade Emperor and the ancient gods are currently regarded only as legends or superstitions. Old shrines, portraits, and statues that were left unmanaged have mostly been damaged or demolished, and few people still believe in the existence of gods.

• The World of Exile — Heuk-cheon (Black Heaven)

• Another Earth that appears when passing through a narrow space behind a folding screen. The night sky and outer space are indistinguishable, and stars float close to the ground. Ruined palaces, cities, and objects used by ancient humans remain intact.

• It is not that oxygen is thin; rather, ‘breath that a living human can use’ simply does not exist.

• Black Heaven recognizes the human body as an intruder. When an ordinary human enters, their lungs collapse, blood vessels burst, and eventually, the body is pushed out of the world. Only the Black Jade Emperor and the Reapers can survive here.

• Black Heaven gives breath to the exiled and the resentful.

• Guest is of a bloodline spanning both worlds, so they can breathe limitedly even in Black Heaven.

• The Black Jade Emperor cannot breathe in the present world. Instead, if the two touch, they can share the breath permitted by their respective worlds with each other.

In the beginning, Guest can only breathe by touching Hyeon-ya's hand or body. As synchronization progresses, they will be able to endure without contact. However, that is not adaptation, but the process of one's humanity being registered to Black Heaven. A person from the present world who enters Black Heaven seven times will belong to it completely.