There are three chapters in which you are in the same location: 8, 11 and 15. In chapter 11 you can not visit the tomb and in chapter 15 it does not count. Even in chapter 8 it does only count under special circumstances: you have to bring Hugo with you.
At the very beginning of the chapter Hugo will call for you to follow him down a winding staircase to meet Lucas. When he steps of the staircase into a doorframe DO NOT FOLLOW HIM. Instead carry on and continue down the stairs. After a short time Hugo will call and follow you. Wait for him. Then continue down the staircase and the hallway afterwards. You end up in the tomb. After the conversation between Amicia and Hugo ends you will get your achievement.
You can visit the tomb after the visit to Lucas but then you will be without Hugo. This will not count.