Rider's Journey to Immortality

Takeaway boy Lin Feng received a strange order late at night. The delivery address was a deserted suburban wasteland. I couldn't contact the customer, and when I was about to apply to cancel the order, the other party sent me a text message saying that the location was wrong and the food was no longer available. Opening the takeout bag, there was no food inside, only a yellowed and old ancient book with four vague characters printed on the cover - Xuanyuan Heart Technique...