This story is an exercise in writing over-the-top plot twists. I may change it. If you can't keep reading, that's not your fault, it's because my writing isn't good enough. Just drop it.
Over-the-top version of the blurb:
Before transmigrating into a book, Ye Yuncheng would react to problems by slinging on a bag and saying, "I'm going to wander the world."
When bullied, he'd cover his face and sigh: "There's a difference between legitimate and illegitimate children. What can I do?"
After transmigrating into a book, Ye Yuncheng responds to a forced marriage with: "I don't care if you're a lord or a general. I'm a master of antidotes. Cross me, and you'll never get your poison cured!"
When bullied, he says: "Gu Qingxiao, the biggest villain in this story, is in love with me! He'll grind you to dust!"
A slightly more serious version of the blurb:
Ye Yuncheng transmigrated into a book and became the illegitimate son of a family of sacrificial extras. He returned from traveling in the southern frontier with unparalleled skill in curing poisons, only to have his own family ruin everything for him.
To change his fate as a sacrificial extra and keep his younger sister from marrying Gu Qingxiao, Prince Pingning, the book's biggest villain, Ye Yuncheng helped her escape the wedding. His reckless family then sent him to the prince's residence to apologize.
Legend has it that Prince Pingning, Gu Qingxiao, is gloomy and violent because he has been sick for years and was born frail. His mother was the emperor's first love, so those who obey him barely scrape by, while those who defy him end up as bones in the ground.
While fleeing and climbing over a wall, Ye Yuncheng unexpectedly ran into Princess Zhaoyun, Gu Qingxiao's sister, who was also running away from her wedding. Since they were both fugitives from marriage, he took the princess along with him.
Who knew the princess would be so sickly, probably because she had been poisoned? Ye Yuncheng cured her poison, but instead of being grateful, she nearly caused his entire family to become sacrificial extras ahead of schedule!
The sacrificial-extra family: "What Princess Zhaoyun? The person who ran away with you was Gu Qingxiao himself!"
A cultured villain isn't scary, and neither is a villain who's crazy. The real danger is when a hot-tempered villain suddenly comes to his senses.
For the sake of survival, Ye Yuncheng helps Gu Qingxiao recover, attending to his every need. He watches him go from a sickly, gloomy boy who coughs blood at the slightest touch to an arrogant man riding with a whip in hand, pointing his sword at the crown prince, the story's paragon of righteousness.
Ye Yuncheng regrets it. He doesn't regret the villain pointing his sword at the crown prince. He regrets that the villain wasn't born frail!
Theme: Courageously follow the path of righteousness