They are journeying to face a demonic cultivator atm. Subplots happen. The subplot about the dragon who made the people infertile and had them kill the women actually has something to do with the main cultivator they are after. Because of the brown haired disciple's memories of his past incarnations, he knows he seemed to be the ultimate target of said cultivator so he's trying to find out who's really pulling the string that he might have offended. He doesn't recall having much to do with demons, so he's also thinking what if it's someone from the heavenly realm pulling the strings in the end, trying to get rid of him. The reason someone targeted him is still unclear to him.
I really really do want to understand and read comprehensively but all these flashback, small episodes, and literally unnecessary things makes it sooo hard for me to understand the story.