Transmigrated as the Demon Lord's Doomed White Moonlight Substitute

A hundred years ago, the genius sword cultivator Xie Wenhan lost the first love he cherished above all others. One thought drove him to fall into demonic cultivation, becoming Youming, a brutal Demon Lord who could destroy heaven and earth. A hundred years later, to kill the Demon Lord, the immortal sects gathered dozens of stand-ins who resembled his lost love, formed a harem, and sent them into the Demon Palace, hoping to poison him in bed. Jiang Sui opened his eyes to find himself transmigrated as one of the members of the lost love's harem of stand-ins. The Demon Lord Youming had a mercurial temperament and was ruthless and merciless. He was a terrifying fiend who killed without blinking. Jiang Sui didn't want to get himself killed, so he teamed up with someone and ran away in the middle of the night. But that very night, he ran into the Demon Lord and watched him burn two stand-ins of his lost love to death without hesitation. When it was his turn, Jiang Sui shut his eyes, accepted his bad luck, and prepared to die bravely. When he opened his eyes again, the Demon Lord had taken him back to the Demon Palace and was treating him to the finest food and care. Everyone around him said Jiang Sui was the reincarnation of the Demon Lord's lost love, the beloved first love he'd searched for desperately for a hundred years. But only Jiang Sui knew that wasn't true at all. The Demon Lord had once sneered right in front of him: "Lover? A mere stand-in doesn't deserve that title." One night, Jiang Sui met the original owner's master in a dream. His master was cold and merciless: "You've been infected with a Seven-Day Lover's Gu. Within seven days, you must have intercourse with the Demon Lord and transfer the parasite into his body, or you'll die." Jiang Sui: I'm done for. Jiang Sui, who'd decided to lie flat, eat his fill, and wait for death, opened the door and was swept into bed by the Demon Lord. As the Demon Lord's hands began to undo his sash, Jiang Sui helpfully reminded him, "I'm poisonous. Anyone who touches me dies." The cold, merciless, brutal Demon Lord gripped his chin. "Is that so? Then I suppose I'll have to find out." Theme: Persevere and survive the quest for immortality.