This achievement can be earned right at the beginning of the last mission, before you leave the ship for Dunwall. The manner in which you earn it, however, depends on how you've played the game up to that point.
It appears that if you've played a Low Chaos game, talking to Megan will simply get her to reveal her story. It's possible you have to have been Pacifist as well, but that's not clear.
If you've played a High Chaos game, she won't tell you (of if there's additional criteria beyond Low Chaos I'm unaware of), so instead you have to pickpocket her key (or possibly, kill her and take it off her body), then use the key to unlock her cabin (off the main room where Sokolv is painting; use the heart to reveal the bonecharms in her cabin if you're not sure where to go) and read the information in her cabin and listen to the audiograph to learn her story.
Either way, the achievement should unlock.