Since it mentions martial arts and demonic cult I'm guessing this is one of those world with cultivation
So my guess, given precedence in other series, is that
She basically has a Yin type energy and possibly with blocked circulation resulting in her being weak and can't cultivate through normal means.
And nobody ever figure this out because it's such a rare condition and she's such a quiet girl that she ends up dying young in the original story.
If this guy can actually save her though, this type of super rare constitution she has is also supposedly able to train faster/beyond normal limit and basically become godlike/actual god level.
Now this is how you make a good short promo. It gives enough background and ends at a point to grab your attention. The description is interesting and combined together it makes me want to hunt the story down and read it.
The amount of promos appearing as ch1 and one shits is still annoying but I will keep reading them. I wish o could say because I was interested in the novel their promoting but really I just don't pay attention to weather their promos or not. Really need a promo tag with all were getting. One shots and promos are entirely different beasts.
Maybe, tbe male lead in the novel actually fell in love with the girl, but the girl died, and then he turns back time, the girl got isekaid, and he try his best to pleased her.... Okay i need to read the novel lmao XD