Fantasy: the Useless Son-In-Law at the Beginning

A fantasy story about a son-in-law who kills the decisive protagonist and a single female protagonist. His teachings reach heaven, but he is hidden in the world. He is as rich as anyone in the country, but his clothes and food are poor. He has millions of men, but no one knows them. His name is Duan Lan, and he is now the useless son-in-law of the Bai family. Let's see how he thrives despite having a useless son-in-law at the beginning. This novel and its characters are purely fictitious. Any similarity is purely coincidental and should not be imitated.