This achievement unlocks at the end of the credits. Just let the credits roll through at the end of the game, and that's it.
NOTE: Some people experience crashing during the credits. If this happens to you, go to your Steam Library, right click on Keepsake, and choose Properties. In the properties window, go to Languages and choose Francais (French), and close the window. Once the new files have been downloaded, run the game in WINDOWED mode(not fullscreen). Play through the ending again, and now it shouldn't crash during the credits, letting you get that final achievement!