I am unsure if these are all random drops or guaranteed or if they are difficulty specific but you need the following collectibles to unlock this story:
Lu Clan Genealogy - dropped from the enemies in chapter 1. I got this as a part of my normal playthroughs
Luxurious Letter - dropped from the boss at the end of chapter 1. I got this as a part of my normal playthroughs
Lu Kunhai's Order - dropped from the enemies in chapter 1. I got this as a part of my normal playthroughs
Once you have all three go to the stand with the blue book at the beginning of the game and unlock the story for the achievement.