Crystal Core

The doctors couldn't explain what was happening to him, but slowly, his body was crystallizing. Then, a portal opened up on the ceiling of his hospital room. He'd seen that one in enough anime to recognize what was happening. It couldn't possibly be worse than the hell that was his life on Earth, not that he was given any choice. Now, those who summoned him expect some type of hero. That isn't Ryan. Ryan wants freedom. He wants to live his life his way. He'll do the right thing when confronted with it, but he isn't out to be anyone's savior. Of course, in a world under a brutal siege and surrounded by powerful cultivators, Ryan realizes the only path to freedom is becoming strong enough to make his own path. For once, the mysterious disease will work in his favor as he forms his Crystal Core. This story features slow-burn romantic interests, world-building, a cultivation system with LitRPG framework, making fun of cultivation tropes, powerful supporting characters who have their own motivations and backgrounds, and an MC who has potential but is still growing in power. Be warned, there is some mild cussing from some of the characters, and the MC undergoes a great deal of personal growth in this book so that he will be in the place he needs to be for the rest of the series.