Rules for Winning Over the Male Lead as an Undercover Villainess: If I Can't Die, What Have I Got to Fear?

[Xianxia + Wild Imagination + Lighthearted + Ridiculous + Sweet Romance + System] I transmigrated into a book and became a spy planted in a cultivation sect. This spy was no ordinary one. You could even call him a success: he had worked diligently undercover in the demon realm for decades, only to kick the bucket just after becoming the Demon Realm's Left Guardian. After I arrived, the System told me to win over the Demon Lord and stop him from self-destructing and destroying the world. At first, I thought I might soon meet the same end as the previous occupant of this body. Then the System told me I had an ability that supposedly made me impossible to kill. As long as I hadn't completed the mission, I couldn't die. Me: ! Maybe I could just lie flat. ＊ When Xie Sangning first met the System, she saw all the features on its interface and thought she could really make something of herself. But later, the System said, “Dear Host, the Demon Lord is too abnormal and could sense the System's presence at any moment. System No. 1003 will explain your mission and then leave. For a detailed guide to the interface's features, please see the System instructions at the bottom of the page.” Then it vanished. Xie Sangning: ! A self-service System? ＊ One day, before Xie Sangning had figured out how the System worked, she was inexplicably teleported somewhere. After a dizzy spell, she hadn't even managed to look around before she was struck down and killed. The people of the demon realm looked on in shock: “The Left Guardian loves His Majesty so much that she took a blade for him!” The Demon Lord, who'd been inexplicably shielded from the blow: “...Who is this person?” Ten days later, Xie Sangning, laid in a coffin, rose from the dead and terrified everyone in the Demon Palace.