To unlock this achievement you must have Matkina in your party at the completion of the game. Matkina can be recruited during the progress of the main story.
Choose the option that involves Matkina and the achievement will pop during the end credits.
Suggestion below (contains spoilers):
*** Spoiler - click to reveal ***After beating the spectre (last boss battle) you can save before heading into the room to have a long discussion with the sorrow. There are quite a few ending decisions that you can make that influence other achievements. Reload your save and check out all the other endings.