Impermanence

The Black and White Impermanence, divine officers of the underworld. They wander the mortal realm as soul collectors and guide the dead along the road to the Yellow Springs. Whenever someone trapped in despair cries out helplessly from the depths of their heart, the two appear before them, bringing a contract full of hope, while leading them willingly into another kind of desperate situation:

"You who are sunk in despair, would you trade your soul for three wishes?"

A frightened, disbelieving voice: "But if I lose my soul, can I really go on living?"

"Spend ten years on the mortal road today, and a hundred years walking between yin and yang in the life to come."

After careful consideration, the answer comes: "I'll sign."

"Think it over carefully. If you break the contract, you'll pay a very, very terrible price~"

P.S. A seemingly gentle, cold-blooded badass heroine x a gentle male lead with dissociative identity disorder

Reader's Guide: Episodic format, with one character gender-swapped. An incredibly sweet married couple, nothing sad, just love! The badass heroine shows tenderness only to the male lead. The male lead really does have dissociative identity disorder, with another personality!

Theme: Good is rewarded and evil punished. If retribution hasn't come, the time simply hasn't arrived yet!