I don't know the whole story about that part. [ He says quickly, apologetically. ]

It happened a long time ago. But he was - he was friends with this cultivator named Song Lan, and something happened to him. Lost his eyesight. So Daozhang Xiao saved him, by taking him to a really old powerful hermit cultivator. He gave his eyes so she could give them to Song Lan.

That's all I know. Sorry. Something happened and they separated again, and that's when Xue Yang found him, used his blindness against him. Tricked him into horrible things he would never have done, otherwise.