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:
Dragon Chronicles Pages - X - dropped from the enemies in chapter 4. I got this as a part of my normal playthroughs
Dragon Chronicles Pages - XI - dropped from the enemies in chapter 4. I got this as a part of my normal playthroughs
Anomaly Report - dropped from the enemies in chapter 4. 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.